When everything you look at and address for a problem is definately right, then you're looking at the wrong thing. So what is the symptoms you keep finding..
Last year I did a fresh set of 915s for a guy who "kept blowing head gaskets". He kept finding an external coolant leak at the track, or after a long hard beating session on the street. It was a crack from the #3 exh port to the exterior surface of the head. The fluid would appear at the parting line between the head, gasket, and block. Sometimes drip a little, sometimes just a stain of green down the block behind the headers. Hopefully it's nothing big.