1997 Dodge Caravan the water pump or head gasket

I have 1997 dodge caravan and it has over heated on me. Was low on oil when it happened. Since then it overheats when under load and over 2 to 2 and a half thou revs. Heats up quickly from cold but if you crack the rad cap open and release the pressure and loose a little fluid it will run a half temp most the time. But give it a little to much power and it will raise in temp again and then bring down the revs and will slowly hit half temp again. Thought it was a head gasket but there is no water in the oil and no loss of power and no smoke? I also notice that the heater is not working properly. I have eliminated the radiator and the thermostat so can anyone tell me what it is.