I have a 1999 Venture. The problem is with the heat gauge and inside cabin heat. The heat gauge will go all the way to the red at times and then all at once it will go back to normal. The van does not seem to be overheating when it does this. You can touch the radiator cap and the radiator is not hot when this happens. The heat will not be hot. It takes forever and then when it does heat up it is like the heat gauge will go toward the red and the heat will get luke warm and then the hand will back toward normal and the heat will be nice and hot. Just dont make sense. It has a new heater core and temp sensor and also a new thermostat. I see no signs of coolant in the oil. I know it can be the intake or head gasket but should there not be signs of that somewhere. Please someone reply with some advice. I have a head gasket set on the way just in case.

 

Sounds like the thermostat is sticking. Check it with a pot of water on the stove with a thermometer. When the water gets to the temp the thermostat opens, put it in & see if it is opening.