Just thinking outloud here... Does the truck have automatic climate control? Since you have no different results from before changing the thermostat, I'm guessing the coolant system is full and purged of air. It doesn't seem like the heater core would cause this. If it does not have a.c.c., try moving the temperature control around just a little bit. I once had a Taurus that when I turned the heat all the way up, the temp. mix valve would fluctuate, but if I turned it down just a bit, it would be fine. Try doing a forum search as well, if you haven't already. Rob