Redford

I'm new to the F150 forums so please be patient with my lack of knowledge. I've owned a F150 with the 4.2 liter V-6 for ten years. I bought it new off the lot. The temperature gauge runs normal for about 20 miles down the highway, then climbs up into the red. The radiator never boils over and I can get the reading to come down to a tolerable level if I turn the heater on (not a pleasant experience when it is 90 degrees outside). The mechanic put a moniter on the truck and said that when the gauge goes into the red, it is only at 237 degrees. The thermostat, sending unit, and water pump have all been replaced and the problem still exists. He says that the gauge must be bad and recommends replacing the instrument cluster. Does this sound right?? Can anyone help me out?