Last Nov we went to a tractor pull at White Pine Tn.we started home and Son turns the heater off,we stop at Hardees for a snack and while he goes in I turn the heater on and it blows cold air,we leave and with in a mile the check engine light comes on and temp gage showes hot and about that time it goes back to normal.We drove on home [35 mile] all was fine.I had the thermostat replaced,Monday it did the same thing again,replaced thermostat again and it did the cool heat thing again today,Talked to Dodge Diesel mechanic and he thinks head gasket[ouch] but don't seem right to me,Anyone else had this problem? truck has 103,000 miles on it.Any help would be appreciated