[schade]

back on topic for a moment ... i doubt its the thermostat as nowadays thermostat's are made to "stick open" as to not let the engine overheat.

Have they done a pressure test on your cooling system? and rad cap? checked to make sure the fan is comming on?

maybe wire the fan to stay on all the time and take it for a drive and see how it runs (thats if the car was overheating when the "!" light came on