Did you replace all the piston ring seals, the big one on the outside of the piston and the little ones that are on the shaft?
Are these steel rings that have locks the hook together where the gap is? I had this problem once with a Yale. Once it warmed up it slipped in forward and that shaft piston ring was worn enough that it had expanded all the way until the lock wouldn't let it open enough to make a good seal. Cold it would go ok but once warmed the thinner oil and leak-by past that shaft seal caused the clutch slippage.