Ideas on what is making this overheat?

I have a 2000 Oldsmobile Intrigue and it is overheating. I have replaced the water pump and coolant overflow tank and removed the thermostat and had the radiator checked for blockages. The radiator is fine. The car will overheat very slowly if you are not above 2000 rpm. I usually have to drive in 3rd or 2nd gear to combat this problem. Also there is no oil presant in the coolant nor coolant in the oil.