another thing you can check out is if the car cools down when you turn the heater on full blast. If a lot of heat comes off the heater it means the water pump is pumping water through the system, and that your thermostat allows hot water to go through. You can test the thermostat function by just removing it and seeing if the car will run at normal temp.

Did you ever overheat the car, like super hot in the red for a long time before you had this problem?