My car is 91 240. I got hit at the back on the driver side (the side bumper fell off). I did not check the temp gauge was ok or not after the hit. However, the next day when I drove, I found out the temp gauge went up to near red when I first completely stopped the car (The car had run about 5-10 mins). Then, the gauge started to go ups and downs according to the hills or flat road in the city. When I continued to drive, the car didn’t have smoke. Everything was the same as normal except the temp gauge.
However, unfortunately, the head light was gone suddenly when i started the car again on the same day.
The next day, I drove to work, the car still had the same problem, however, during the 45mins driving on the highway (hill or flat road), the gauge needle maintained in the mid point, which is the same as before the accident. After I left the highway, it started the fluctuation again. On the way home on the highway, it also maintained in the mid-point, and when I drove in the city, it still has a little bit ups and downs fluctuation, but it seems getting more stable. Would this situation go away itself? Does this problem can be fixed as the “temp comp board” message? Would the hit cause the clogged, wire loose condition or it just happened because of the aged? I would like to have a second opinion before doing the expensive radiator repair as a mechanic told me. He said the radiator is half boiling and half very cold. Mostly the radiator has been clogged, so the temp gauge goes up and down. I remember the coolant has been changed three months ago only. Please Help!

Thanks in advance.