95 Grand AM overheating

Hello, I am looking for some help on a 95 Grand AM GT with a 3.1 V6 that is overheating and I have not yet found a solution. The temperature guage will fluctuate from 150 deg to max and back down to 150 within a 2 mile stretch. I have replaced the thermostat, and the water pump. The car has a new radiator, installed through the previous owner. I have bled the air out on numerous occasions and it is still overheating. I do have heat and the car runs fine. It does't seem to be missing. When it overheats, the water comes out of the resivoir cap. I have no problem changing the head gaskets and heads if thats what it takes. I just do not want to do that if that is not the problem.

Thank You for any help that you may be able to give me.

Before going to head gasket, check the easy stuff that will cost you a little time and less money: Take out the thermostat, place in boiling water, does it open right up? If not, you may want to try another thermostat. As the car starts to overheat, are the electric fans coming on to pull air over the radiator? If not, you will want to start diagnosing that problem. Did you flush the system when you changed the water pump and thermostat? If not, that's an easy place to start--flush it out good and refill with the right mix of water and new coolant.

I'd have the cooling system pressure tested. It sounds like a bad intake manifold gasket or head gasket to me. A pressure test will pinpoint it for you.