2002 Chevy S-10 Temperature gauge high but not in the red

I have a Chevy S-10 Crewcab 4x4 (Chevrolet calls the model T-10). After driving a mile or so the engine temperature gauge goes way beyond normal. The gauge should run at 210 degrees, even in idle. Mine is going beyond that at 235-240 degrees, to the next large notch on the gauge. After it reaches that temp, it stays there and never goes down. Burning and rotten egg smells come from the under the hood when it reaches this temp. My truck manual says I can disipate some of the heat by turning on the heater in the truck, so I tried that. But it seems that suddenly no heat will come out, only cool air. So it also seems that I can not heat the cab area now. I got the oil changed a couple of weeks ago and all of the fluids were topped off as well. My truck still has the original orange antifreeze in the radiator and the mechanic said it was at the full level. I am worried about continuing to drive the truck at this temp. Before I take it to the garage, I wanted to get your opinion. Do you have any idea about what the problem is?