As I recall, didn't you tell me that this has only happened on track, at speed? The more I think about that, the more I think it's probably a PP issue. Is this the same one that was in there when you bought the car from Bob?

FWIW, my car had a stock type clutch when I started racing it, and it wasn't long before I started getting slippage about 15-20 mins into a race. While I know our cars don't make much more power than stock, we are still abusing them well beyond what the stock components were designed to handle. When I built the engine I replaced the clutch with an ACT heavy duty plate & 4-puck disc, and I've never had a bit of trouble since then - 5+ years and maybe 15-20 events later. YMMV