The same thing happen to my wife's 98 jimmy. It started happening when the temperature was over 95 outside and with the ac on. I change the coolant temp sensor, actually i relocated the sensor next to the thermostat housing instead of the side of the block, got a new pigtail harness, put in a new radiator since it was the original and it was pretty dirty, flush out the system and while i was their cleaned up the egr valve. before I did this running temp was about 200 to 210, now it stays at 195 to maybe 205 on a very hot day and the ac blasting. Florida weather is a bitch. I hope i was able to help you out and goodluck.

 

Nope, mine was a bad ICM.

All good still after 10 days. I call this one SOLVED.