heater on 1997 Ford F-150

heater is on and starts blowing cold and temp gauge goes hot,turn off control to heat/air,temp goes back normal.can turn back on and works fine untill next time

2 answers
hey dude i'm afraid that you have a blown gasket. The thermostat is definately bad, but if you keep driving when the gauge goes hot then you will need a new engine(will warp the block)

Super Mario Bros.
My inital thought is sticking thermostat, but I think you'll need a hands on diagnosis to help with this one.