The "cold at idle, hot at speed" scenario you describe is exactly what I had on my '92 Trooper at about 150K miles. The problem for me was a "tired" water pump. It wasn't leaking and had no bearning noises, but it was not circulating as well as it should have been ... it took >1300 RPM's to circulate "good heat" through the cab of the truck. With the mileage you have on the vehicle, you could be looking at a tired water pump. HTH ... Fred.