Overheating?? What could be wrong??

Head gaskets would be my first guess, but as Ragged says above, make sure your fans are turning on. Check the fan relays first and then hotwire the fans to make sure they run. Have you checked for exhaust in the coolant? You can purchase a tool here that can help you to determine if you have exhaust in the coolant:


If you determine there is no exhaust in the coolant, then I would do a complete system flush. Sometimes a clogged heater core can slow down the flow enough to cause the type of overheating you are talking about. Once you are in high gear, the engine rpm slows down which also slows down the water pump speed, and at that point, if it is, in fact, a heater core, it might not have enough to overcome a poorly flowing or clogged heater core.

Coolant doesn't have to be present in the oil to have bad head gaskets. They might be leaking coolant directly into the cylinders or outside to atmosphere and not going into the oil gallerys. Does the car smoke at all? Can you smell antifreeze at the muffler tips? Are the tips greenish colored around the outside edge? These could all be signs of head gasket failure without it pushing coolant into the oil.

I would have a pressure check done on the entire cooling system and listen very closely to see if you can hear it in the intake. If you can, then it's leaking into the cylinders and not the oiling system.