I have a 99 G20 with 130K. Bought it new, and it's been a great car. Recently, it's been overheating in the hot central valley temps. I can turn the heater on to 85 and the temp goes down, however once I go past 74% it starts to overheat again. All my fluids are good, I've replaced the thermostat and even had the radiator flushed by dealer with no remedy. Any ideas?