03 Dakota thermostat problem

about a week and a half ago, i took my Dakota for a 250 mile trip. about halfway through the trip, the engine began to overheat, and i had to stop a couple times to let the engine cool down. i continued on, put the heater on full blast, and found that i was able to get the temp to drop by kicking it into passing gear. i would have little to no heat coming out of the heater core, then id hit the gas and raise the RPM's. heat would pour out of the heater duct, and the temp would drop, only to rise again shortly thereafter. it seemed pretty obvious to me that the thermostat was bad, because it was behaving as though it was closing too soon and opening too late. so i got to my destination and the thermostat was indeed bad.

i had it replaced, but exactly one week later, the same thing is happening again. any ideas?

When its to operating pressure feel both top and bottom radiator hoses they should be hot. Feel the hoses they should have an internal spring preventing them from collapsing. Check the belt for the water pump and make sure it has the proper tension.