My coolant warms up normally when I start driving. About 3-4 minutes later the heater ducts start blowing cold and the temperature gauge drops rapidly from normal to stone cold. After 1-2 minutes the vents start blowing warm again and the temperature goes back to normal. Always happens at warmup, then seems okay after that. Happened last year and I replaced the overflow cap and thermostat, no luck. A local shop said it was a blown head gasket. Had the coolant sytem pressure tested, no leaks. Changed the radiator cap again, and no problems. This time I just changed the radiator cap with a new Motorcraft from the dealer, no problems for 2 trips to work, now it's acting up again. Coolant level seems to be stable. Could this be a water pump? I'm thinking it's not the thermostat or radiator cap.

 

everything I see points to water pump or blockage

 

im leaning towards a sticking thermostat, that hangs open after its first opening