Have a 1989 4.0 has 265 thousand miles. I have replaced everything I can think in the cooling system and still have an overheat issue. When running the temp gauge will cycle down in tempture one to three times then stop cooling. Temperate on gauge reads around 220 when water starts boiling out of reservoir. Have done several coolant system flushes with a few bottles of coolant system cleaner I got from orielys. Any help would be greatly appreciated.