On my 01 F150 crew, 5.4l, 4x4, took 5 of us up a tall steep mountain. Outside temp was about 109, AC blowing and had it locked into 2 nd gear. Noticed temp gauge climbing up...and up....finally get to the top of the mountain, aimed the towards the wind, popped the hood and let it sit for a spell. Cracked the radiator cap just a little until it got close to spewing out. Temp dropped, when back down, all is good. On the way home, flat road, AC going, around 110 degrees, saw the temp guage go up, then dropped back down to the normal position. I've drained and replaced all the antifreeze about 5K miles ago. Fan works as it should. So, I replaced the thermostat and about half the antifreeze, even though it was all clean. When it started running hot that second time, I'm thinking the thermostat was maybe getting stuck or not working properly because I was driving on a flat road, engine running around 2K rpm, plenty of air moving through that radiator, then seeing the temperature guage drop. Anyway, was wondering what ya'll think. Don't really drive the truck a lot, it's not my daily driver, especially at these gas prices and 13 mpg, on a good day, downhill in a tornado. Also, being that the thermostat is factory original, just felt that was the right call. Truck has 113K on it.