Defrost, vent and or heater will blow warm then.

Defrost, vent and or heater will blow warm then hot air until the car reacheds normal operating temperature then it only blows cold air- a feew times it has switched back to hot for a few months, have replaced thermostate, flushed heater core- was clean- unable to find a heater control valve- not sure it has one or not- autoparts stores sell one- unable to located it- don't think it is a blend door because it does blow hot for awhile- unable to find a vaccum leak.