If this happens again, try this: Set the cabin heater on full hot and full fan. That makes the heater core perform like a mini-radiator. If you see the temp gauge needle drop, then there is a lack of cooling capacity in the system. Old plugged radiator, maybe something else.

Also, tranny in gear might heat the ATF a little hotter than when in neutral. With the ATF cooler inside the radiator, and fan-only air flow, that might account for some of the heating up.

Definitely check/replace the thermostat. Labor to check = labor to replace, and they are inexpensive. If the removed unit is OK, you have a good spare.

Good Luck,

Bob

:>)