occasional overheating

[tls4every1]

Hello all. 2007 Sport Trac 4.6 L. Sport trac just started overheating randomly. It would start getting hot then cool down to normal in a few seconds. I figured it was the T-Stat but I just replaced it and no change. Seems to happen more frequently on heavy acceleration or when AC is on. Coolant is full, no leaks. Water pump? Fan? water pump is original equipment I believe. Fan ASSY is about 10 years old. Complete scenario was I was pulling our camper this Friday. noticed it getting a touch warm 30 minutes into the trip. slowed down to 60 mph on the interstate. drove two more hours and it happened two more times. slow to get hot but came back to normal in seconds. Checked fluids at the campground and all looked good. Drove home and it was fine for the first two hours at interstate seeds and in traffic. then started again. never got to red warning lights but came very close a couple times. Now does it not pulling anything. I replaced the t-stat this morning and no change.
Thoughts?

 

[HeyNiceSporty]

The truck is running OK otherwise? No engine codes? No possible lean conditions?

Do you trust the sensor giving information to the gauge? Check coolant temp with another source?

Have coolant changes and flushes been done on a regular basis?

When you changed the T-stat, did you run the heater, and make sure you burped all the air out?

Fans tend to do their job at lower speeds, there should be enough airflow at highway speed to keep it cool. Is all the shrouding in place? Spoiler under the bumper? The piece that hangs down in front of the radiator behind that?