1986 thunderbird turbo coupe over heating problem

105000 miles, great clean smooth running car with one exception. The car over heats when driven more than 15 minutes. The temperature gauge is all over the place but sometimes showes correct normal tempurature.with no coolant loss, no water in oil or oil in water (radiator) replaced all hoses, flushed at home, radiator, thermostat (twice from Napa), water pump, radiator cap so far and have burped the system 4 times. I appreciate any help that is available.


At this point I would test for the presence of hydracarbons in the coolant. Many auto parts stores have kits to do this.

You sure it’s not just a flaky gauge?
What other signs of overheating?