heating problem 96 mustang 3.8 v6

i have a 96 mustang with a 3.8 v6 and it will blow warm heat then it will blow cold air and then it will blow really hot air after that,i had a new waterpump put on,new top radiator hose.and thermostat.could it be a bad thermostat or? the head gaskets were replaced last year.someone please help me out.thanks larry

I'd say the cooling system has some air pockets in it. Check your radiator and overllow tank to see if you need any more antifreeze.