[ChevLoRay]

That's the thing about the mechanical thermostat....they stick without warning. When they stick, things get funky, fast. My money is still on the mechanical thermostat.....get one, pull the thermostat housing, the thermostat, and clean all of the gasket goo off of it and the intake. Install the new thermostat, being careful not to install it upside down....it has happened. Be sure to use a new gasket on the thermostat housing. Some are self-adhesive, some require adhesive. When you have everything ready to bolt down, don't use too much torque on the bolts if your housing is aluminum. Some are, some are cast iron. Make sure all of the hose clamps are tight.