I can back-up Jeff's advice.

In the first house I owned, I had exactly the same experience with a water heater that was 2 years old. Temp safety switch
tripped for "no reason", I reset it, tripped again a couple weeks later, then it became more frequent. Along the way, I
noticed that the hot water was intermittently a lot hotter than normal.

Flipped a coin and decided to replace the upper thermostat which did not fix it, so I replaced the lower one which *did* fix
it. Like Jeff said, they're cheap and if I have the same experience again I'll just replace both.

Eric Law