I think I'd rent a pressure tester from AutoZone and see if you're blowing gasses into the cooling system. As often as this puppy has overheated, you may have a head gasket leaking. It sounds like you are overheating and not a short in the wiring.
When the engine is cold, you might want to pull the spark plugs out one by one and see if they all look alike. If you're burning coolant, one will look a bit different. Are you losing coolant?