My friends 2006 Aveo started overheating about 2 months ago. Started out just a little so I wasn't worried about it. However it started getting worse. Now it climbs to about 3/4 on the gauge and the A/C gets warm. If the A/C is left on it continues to climb. If you turn the heater on though, it rests about 3/4. A year ago I replaced the thermostat because the OEM plastic housing was cracked, so I didn't think that was it. I have since replaced the water pump. I pulled the radiator and cleaned it out, it is perfectly clean inside. Coolant is clean, and full. Also performed a hydrocarbons block test to check for a bad head gasket, twice actually, and it passed both times. It's got me beat, I have no clue what it could be. Any ideas?