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!