running hot

I have an '04 Yukon that has recently started giving me the warning that the engine is running hot (along with the temp gauge pointing close to the red zone). I can get by when it does this by crusing around 45-50 mph and it will cool just a bit to at least stop giving me the "hot" message. But, then it will start all over again within ten minutes and be back in the hot zone again. I took it to a GMC dealer and they "could not find anything wrong with it." It seemed to start a few weeks ago after a 250 mile trip pulling a small trailer (10 feet long). Any ideas on what to check? Thanks!

Thermostat stuck in the closed position? Assuming that they checked for that already, I'd guess a bad temperature sensor. I'd also say a bad temp gauge but I don't think that a bad gauge in itself would necessarily trigger the warning message...