Car intermitently over heats

Last month my car was overheating when I was coming home after work. I tried turning on the heater to keep it cool, but after driving less than a mile the car would over heat again. I check the fans, check coolant, check oil, and everything was fine. I left the car on the side of the road and came back the next day to remove the thermostat so that I could drive it home. Even with the thermostat off the car at around 5 miles the car would start overheating. Two days ago I drove the car to the dealer to let them diagnose the problem, when I was driving there, at about 1/2 mile the car started overheating, so I stopped the car and let it cool down before driving again. I got it to the dealer and they told me that I have a blown head gasket. I picked the car up yesterday. Drove it around 10miles to my brothers house so that I could try and fix it. I was surprise that the car did not over heat, so I left the car on turn on the A/C to max and left the car running for 30mins. Revving up occasionally to 2000rpm and I could not get the car to overheat. Do you have any idea what the problem may be? Thanks.