Sounds like a failing radiator or water pump. The coolant just cannot keep up with the heat generated. Using an IR thermometer to check the temperatures across the radiator would show if this were the case.
Another cause of this same issue would be a partially plugged exhaust. That traps the heat in the engine and slowly overwhelms the cooling system.
An exhaust back pressure test would show if that was the case.
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Sunday, October 8th, 2017 AT 2:42 PM