I love my Jeep. I've had it since I drove it off the lot new. That
was 140k miles ago, and it still looks and runs nearly the same as my
first trip...Except

The last few months it has been running hot once in awhile. It is
generally on very hot and humid days, but in the past it handled them
fine.

I suspected the thermostat, so I replaced it and the housing, power
flushed the cooling system (it needed it). It seemed to do the trick,
but then again it cooled off outside so probably was a coincidence.
It happened again.

I checked the fan clutch and it seems fine, I cleared out the radiator
fins, checked tightness of the S-belt. All good. I am thinking maybe
it is the sending unit, because the way it overheats is very odd.

It will go up to 210 and just sit there like it is supposed to. When
it overheats, it doesn't just gradually increase, it snaps up to just
under the red line and the "Check Gauges" light comes on. Then a
couple minutes later it will snap back down to 210. Then a bit later
it will snap back up again to under the red and then snap back down.

No noises or noticable changes in performance. I was wondering if
this is some kind of computer symptom of something else overheating,
but I can't find anything on it anywhere.

Any ideas?