ahhh right on about that gasket donbrew - GM changed to the non-gasketed movable valve in them a couple years ago. I Wonder why?

 

Turned out it was a faulty temp sensor.
The reinstalled thermostat began doing the same thing the next day, and I noticed strange temp readings when the gauge was working, from 156 to 180 in a matter of a few seconds. Had the system flushed and refilled with no change. Temp sensor was $20 and I installed it in less than 20 mins. Problem really, finally solved.
It is not always the thermostat!

 

Glad you got it fixed at last. That rapid temperature change definitely pointed to the sensor - I had a sensor fail exactly like that in a Dodge - would shoot from normal to top out on massive overheat in a second or two.

 

problem solved awesome!!