i have a 97 f150 4x4 4.6. the problem is you start the the truck drive it for a couple miles or so stop at a store turn off truck is fine up to operating temp come out start it back up drive back home almost home the temp slowly drops back to cold on temp gauge no heat in the cab anymore get home just fine sit with truck at an idle watchin temp gauge then boom totally hot buy gauge still no heat in cab turn truck off and restart back to normal operating temp but then back to hot rev the truck up blows some water out the radiator cap then its fine from then on drive anywhere normal sit at an idle for hour or so just fine wake up the next day same thing again burps water one time back to normal all day normal next day repeat same thing but if i get in truck and drive it and not stop no overheat normal. makes no sense to me please help i have replaced radiator cap friday, today sat the 3rd replaced themastat, and 7 months ago replaced clutch on fan anyone have any ideas thank you guys.

 

Are you "burping" the system? -evacuating all of the air when you change the thermostat? I'd suspect air pockets in the heating system by your description.
Look for evidence on the passenger floorboard for coolant just in case as your body style was notorious for heater core concerns.