The reason I asked about the t-stat was because of racing at local short tracks. I've seen alot of cars without stats overheat. We always cut the center out of them to slow the water down and keep it in the motor just a little longer. We found that not enough heat was getting transfered to the water at high rpms. So I was thinking maybe it was stuck open or maybe your old radiator is getting gunked up and not getting your coolant temp down before it goes back to your engine. Hope you find out what it is.
I hate to even bring this up but did you narrow it down to your coolant system and rule out a head gasket?
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