I have the exact same problem and was told by a mechanic that these vehicules frequently leak from the head, and that the gases escaping from the combustion cause the thermostat to malfunction. Either way, what he recomended was take it to an emmissions place and have them check the radiator fluid while the care is hot and running to see if there are exhaust gases in it. Heating a high speed then would be a sure sign of leaking head gasket. I'm gonna try it