Man, I'm thinking it's gotta be in the head gasket. The blowby into the crankcase usually makes me think it'd be rings, but you weren't burning oil or anything, nor was it down on power. Did you ever get the temp indication working? In other words, could you be sure you weren't overheating at all?
On the upside, now would be a good time to look into higher-comp lighter than OEM pistons