Hi,
I have a 2002 Dodge, I am the only owner, and 2 weeks ago for the first time the check gage light came on, while I was driving on the interstate. I pulled off and let it cool down and it was fine for about 4 days, then on my way to work on the interstate, the check gage light came on again, this time when I stopped, there was steam coming out from the hood. My husband came and got the truck, took it in and they replaced the thermostat, measured the heat, visually checked the radiator and re filled it. After scratching their heads as to "why". My husband took off towards home, the gage went up about 3/4. Today we took it for a ride going about 55 -70 miles an hour with the A/C on it hit the black line, no light and it didn't overflow, but when we slowed down the temp dropped quickly.? We then went down a bad dirt road, put it in 4 wheel drive and had the A/C cranked, and the temp gage stayed normal.

What is wrong? All suggestions will be greatly appreciated as we and the mechanics are baffled.