Why are second terms usually trouble for POTUS's?

OK. Hear me out on this. As of the Twenty-Second Amendment, proposed in 1947 and ratified in 1951, we no longer allow a person to serve more than two terms as President of the United States. We have term limits on POTUS. Starting with Harry Truman, the maximum became two terms. Franklin Delano Roosevelt was the last person to serve more than two, and only four presidents have been re-elected since then: 

1. Nixon elected 1968, re-elected 1972, although he didn't make it through his second; 
2. Reagan elected 1980, re-elected 1984; 
3. Clinton elected 1992, re-elected 1996; and
4. Bush the younger elected 2000, re-elected 2004. 

The other presidents since then either died, did not choose to run, or ran, but couldn't manage to be re-elected after their first term. But is that second term really a good idea and what costs are we willing to pay for that second term? And, have those second-termers really had positive, worthwhile experiences? Were their second terms worth the trouble?

Answers to these questions have been posed by others, but by way of answers here, first look at what's happened to all four of these two term presidents since Nixon. In each case, something occurred during their first terms, or came to light during their second terms, that made their second terms at least problematic and in some cases down-right scandalous. 

• For Nixon it was his own hubris and paranoia that brought him down by August of 1974, half way through his second term. 
• Reagan was re-elected in a landslide in 1984, but by 1986, he and his administration were knee-deep in Iran-Contra. 
• William Jefferson Clinton was re-elected in 1996, beating Robert Dole by almost nine points, but by 1998, his fondness for cigars and interns, the hatred so many seemed to have for him and his wife, made his second term an affair to forget. 
• And by the beginning of George W. Bush's second term, his ham-handed attempt to "spend political capital" on privatizing Social Security, an idea it turned out no one liked, was akin to writing a political check that bounced almost immediately. This and two unpaid-for wars assured his second term would be cast poorly by historians. 

But why did the second terms of these men end up in so much struggle and strife for them, for their country, for us? Was it just some mistake they (or someone in their administrations) made that mucked things up, or is there more to this? 

Each man suffered in different ways and to differing effects, but each suffered, each was made to suffer, of this we can be certain. Honestly, just how much can any one person really do if he or she A) continues to remain visible as a target for enemies during that second term, and B) spends a not-inconsiderable portion of the first term running for the second? 

And how much does familiarity possibly breed contempt as that hard-fought second term unfolds? In this era of the Internet and the World Wide Web, doesn't that contempt breed even more quickly and more ferociously? So what's the solution? What can help future presidents from having their second terms end so badly? 

It's such a simple solution: eliminate the second term. Here's how. 

Make the time that any one person can serve as POTUS a single five-year term.  The election is held in November, and the inauguration occurs in January when the president starts working, just as now. But be honest, in a 48-month first term, how much time does anyone really spend doing the things a POTUS is supposed to do before he (and eventually, she) has to start campaigning again, assuming he or she will run again? Were you to listen to critics, this time gets shorter with every new president. Conversely, most presidents will start their "last" campaign well over a year before the second election --- George W. Bush announced his candidacy in June 2003; Barack Obama announced his candidacy in April 2011 --- so let's call it an average of 16 months of campaigning before that second election. In a 48-month first term, therefore, that's fully one-third of the time spent on the stump and off the clock, which means the actual work of being president (i.e., sans last-campaign distractions) is getting done for about 32 months, or a a little over two-and-a-half years (2.6666 years, to be exact). 

Now, if you add 32 months of the first term and 32 months of a potential second term, you come up with 64 months, or five-and-one-third years (5.3333 years, to be exact), so let's call the single POTUS term five years: from January of Year N to January of Year N+5, with the time between the preceding November's election day and the following January's inauguration day the time needed to get up to speed, just as it's intended and spent now. 

Think about the benefits...

• The time and effort and distraction of having to campaign between terms? Gone. 
• The time wasted not actually doing POTUS-related work? Gone.
• The political gamesmanship that always occurs when a sitting president legally and rightfully campaigns but is criticized for doing so? Gone.
• The need to raise that massive second-campaign cash? Gone.
• Once again exposing oneself to the potential for special interest influence brought about by these gobs of cash? Gone. 
• The extra time spent being a target for enterprising enemies to claim something that disqualifies a person in that position? Gone.
• The effects of second-term PAC money, thanks to the Supreme Court, which will have a larger and larger emphasis going forward? Gone. 

The person who is elected to the single five-year term is free to operate in a climate based almost purely on the expectations generated for that one election, and doesn't have to think about being re-elected, feel obligated to special interests intent on that success, or pressured by special interests intent on that failure.**

In other words, if promises are made and kept, great! Those who voted for him or her are happy, and although those who didn't won't be, but they won't have to wait too long to try to regain power. Who knows, maybe the next person will continue that good work, assuming it is. 

But if promises are made and broken, then no matter: that POTUS is gone after five years anyway, guaranteed. Detractors don't have to feel frustrated that he or she might run again, and supporters don't have to feel conflicted about deciding whether or not to vote for him or her again. Fewer undecideds might be the result. 

Everyone wins, but the biggest winner is the American public. We do not have to be bombarded with elections every four years; instead, they'll occur every five years and there will be more new people running each time, giving more people a chance at the job (who'd want it, I say, but... some do), and allowing fresh ideas to be aired every five years instead of every eight, potentially. 

The difference between the first four-year term in the current system versus a mandatory single five-year term in this new system, is only one year and includes the added benefit of ensuring that someone who is unliked will serve only one term, and not have the chance to force that second term via obscene PAC spending and a 24/7 barage of attack ads. This could also make third and fourth-party (and more) candidates more viable, by ending the gridlock of the current two-party system. With each five-year cycle, primaries would be held for all party's candidates, not just the party out of power during that first term. What's wrong with a far more level playing field, far less money attempting to influence dubious outcomes, far less time spent making promises, and far more time spent doing the job of POTUS?

Look, I'm sure I've probably missed something here, like term-limiting congress somehow, or something I haven't thought of (it is late after all). But I'm just tossing this at the wall and seeing what sticks, so where's the harm?

**Now here's where disagreement might arise. Some might ask, what's to prevent a total wing-nut from doing something truly damaging in that single term? Well, what's to prevent that now in either term? Those same checks and balances, that same oversight, will still be in place. There's always a way to game just about anything, but has that ever stopped new ideas before?