I had the very same problem with my 1995 Grand Am. I changed out the thermostat, and heater core, check to see if the cooling fan was working, (it was) and still had the same problem. I have a good friend of mine and great mechanic check it out because I too was stumped. It ended up being that the head gasket was cracked. Even though it showed no signs of a bad head gasket. Coolant in the oil, etc.
I had him change out the head gasket for me since he had to tools for it and have had no problems since. Though it will heat up while idling, but will cool down once driving getting the air through the motor. It does NOT over heat anymore.
It was a last resort having the head gasket replaced FYI.
Saturday, February 18th, 2006 AT 4:16 PM