I have a 97 F150 4x4, 4.6l V8. It only overheats on a cold start. The temp gage goes to hot and the fail safe light comes on but after about a minute the gage goes back to normal and the light goes out. My cap does pop and I lose a little coolant from the res. bottle. I have changed the t-stat, water pump, radiator hoses, pressure cap, temp sending unit, cyclinder head temp sensor, and have flushed the radiator. Also I have noticed that my heater will blow cool air until the temp gage goes to hot and starts to return to normal then my heater will start blowing warm air. Could my problem possibly be my heater core or heater hoses.