Have a 1990 Jeep Cherokee with the 4.0 and an automatic. Question is.. If I start it, and let it warm up, it'll get to a temp of 210. Will not over heat. Then when I start driving, it will start fluctuating between 210 and 165. Whether the heat is on or off. Whether the defrost is on or off. The coolant level is fine, I just did a thermostat on it yesterday night, refilled and bled the system. And it's still doing it. If the heat is on, when the temperature drops, the heat inside the truck is noticeably cooler so that leads me to believe it's not just the temperature sensor. Any ideas as to what would cause this? Belt is tight and all, not sliding around on the water pump. No coolant in the oil, no missing coolant at all. Just looking for ideas as to where to begin before I waste a bunch of money on parts I didn't need.

Thanks in advance,

Jer