More of my $.02

1. I echo Bigfish's question - is it really hot? Does the oil pressure drop?

2. What if it is a scale build-up that has blocked a part of the radiator or the cooling system? This could be a electrolytic or chemical reaction that would not show up in coolant flushes. A slow build-up could have finally caused it to go critical.

3. Have you tried Water Wetter to see if that helps?

4. I still think you should try it without the thermostat.

Good luck again!

Chuck