When I crank the engine, the temp gauge goes up slowly to operating temp (mid range). About a minute after that, the needle rapidly climbs into the red and stays there. The engine still runs fine, the upper radiator hose is warm but not too hot to touch, the lower radiator hose is cooler than upper, the coolant level is midway between min & max in resevior. I have recently replaced the thermostat, coolant temp sensor, temp compensator board, and all belts (including timing). I just had the head rebuilt, the radiator is only 2 yrs. old, and the water pump is 1 yr. old. The problem is - where to go next? Bypass the temp compensator board? Have the coolant system pressure tested? Have the radiator serviced? Other parts I've overlooked? ANY advice would be greatly appreciated.