I have an automatic 2004 Honda Civic coupe. Its been overheating. I bought it in December 2015. Within 90 days, it was running hot. I took it to a shop and they said there was an air bubble in the system. They got the air bubble out of the car, and it ran smooth. Fast forward to July 2017. The car starts running hot after running 300+ miles on highways. I limp it 30 miles to a shop (mostly highway miles) and the shop says its the fans. Those get replaced. Car runs OK. Approx 240 highway miles later, the car starts over heating again. I turn the heat on full blast, and drive on the highway from Knoxville, TN to Atlanta, GA. I go to a shop. They says its the thermostat. It is replaced. The car runs fine. Fast forward to two weeks ago. The car starts running hot again. I take it to a shop. They bleed out a bubble they found, and replace the water pump just to be safe. At no time during this mess did the car act up, no steam, no performance issues, etc. I asked the shop if they found coolant in the oil, or vice versa. They said no. I took the car for a little mountain driving tonight and about 1/3 of the way up the mountain the car starts running hot. I turn the heat on and turn around. The fans aren't blowing hot for the internal vents in the car. The moment I pushed the defrost button, heat started coming out. After 90 seconds, the car was a normally operating temp. I limped it home to Franklin, NC to Atlanta, GA. This pattern happened a lot. Once I got the car on the highway, it would cool off to normal within 2 minutes every time. I don't think its a head gasket, because the fluids aren't mixing and I have no performance issues. The coolant levels are fine.
Does anyone know what the problem could be? Note: I've seen similar posts but none with the car cooling so quickly and I've ruled out most major components.