Overflow resorvor filling up
 

1998 f150 with 5.4 has an issue with the overflow filling up and spilling antifreeze out the overflow. When this happens the temp gauge goes to zero and the heater stops putting out heat. The system was back-flushed recently, new thermostat and according to the shop they tested the cap. Now if i leave the cap off of the overflow reservoir the heater works perfect with the heater putting out all the hot air I can stand. Also the temp gauge reads right where it is supposed to. When I shut off the engine is when the reservoir will overflow if I have the cap on the reservoir. My question is could this be a bad cap, plugged overflow hose back to the radiator or a defective thermostat?

Head gasket will do this by allowing high combustion pressures into the cooling system, forcing it out of the overflow.