About two months ago I hit a cow. It pushed the radiator back into the fan, cracking the back of the radiator. When the body shop fixed my truck, they replaced the radiator and fluid. Also the thermostat had to be replaced, it had stuck closed. Before that, the temp gauge, when the engine was fully warmed up, would stay almost exactly at the midway point. For the first few weeks after I had it fixed it was reading the same. But it's the middle of winter, and the temperature had been in the 40's and 50's. Last week it got into the 70's. I noticed that when full warmed up the needle would be just above the midway point. But when I got out, then got back in and restarted the engine, the needle would be 3/4ths of the way to the top of the gauge. When I would start driving again, the needle would go back down to the middle point of the gauge. I thought that the fan might not be working. But I checked it while the engine was running, and it was turning. So what could be the problem? I think that the body shop may not have mixed in enough water with the anti-freeze. But I'm not sure. Thanks for any help or ideas.