could it be the actuator blend door motor
my heat works good then it turns cold then back to heat what couls it be? on 1999 Mercury Grand Marquis
I do agree with the other posts but make sure you also check the vacuum supply to your hot water control valve, if the vacuum line has a leak in it the valve will open and close as you accelerate and decelerate causing the heat to come and go. This leak will also cause your A/C to come and go with load differentials.
fords are bad for the heater box door breaking
could be a blend door but goodguy is correct about the fords and the doors breaking
REPLACE THERMOSTAT AND BE SURE IT IS "FULL" OF COOLANT!