1998 Chevy Astro Thermostat

First of all, is it truly overheating and blowing antifreeze into the overflow reservoir and really getting hot or might you have a bad temp sensor. I suspect that it is really overheating. My first thought would be the water pump, but they usually start leaking through the little weep hole on the pump body. Next I would check for a blockage in the exhaust system, such as a plugged up catalytic converter. Can you feel good exhaust flow coming out the exhaust pipe? If you run the engine at night, does any part of the exhaust system glow red? If it was an engine part bad making it get that hot, it would make terrible noise.