Overheating while driving

[Kimmer]

I have a 2003 Grand Am 3.4L. Normally the temperature sits right between 200-210. I replaced the reservoir, because it was leaking a little. It still overheated. Today I replaced the thermostat. All hoses are sealed and clamps are good and tight. No visible leaks anywhere. Fans go on when temp hits 240. I let the car idle for about 45 minutes. The temp sat right where it should. Still overheated when driving (within the first mile or two). I talked to a mechanic and he said it might be an internal leak, if nothing was visible. I put a bottle of Bars Stop Leak in. Followed the directions, still overheating when driving, but not in idle. When it overheats, it pushes all of the coolant back into the reservoir, which causes it to overflow out. I am at my wits end... Can anyone PLEASE help?!

 

[motorhead'n'black]

Is your fluid clean, or is it oily?
The radiator could be mixing tranny and anti freeze.
Also check your water pump.

 

[melsg5]

arent the fans supposed to kick in at 210 not 240, try replacing the sensor.

 

[planenut_2000]

A small head gasket leak on cylinder #2