Scotty three years ago a flushed and put new coolant in my truck it had 72000.  This was in the summer time and I didn't check the heating at the time.  When cooler weather came, in the mornings i would start the truck turn defrost on and about ten minutes windshield would clear.  Now it blow cool air, not ice cold but not warm.  Once i put the truck in drive and begin driving it starts to blow hot air, and when I come to a stop it will go back to blowing cool air.   First I thought maybe it was the thermostat so I put a new one on and it still blew cool air.  So I took the truck to dealership and said I had an air bubble so they replaced the coolant and told me that should do it.  It didn't.  One other thing I had my truck parked going down hill, and when I tried to start it, it wouldn't I let it cool down and removed the radiator cap and started right up.  That was two years ago and winter is heading this way I've learned to live with it but  any suggestions please not the heater coil.  At this time i've got 101000 miles on the truck.. Thanks Steve