My issue was similar, I'll recount the tale in case it helps although the issue was different (sort off) Mine is/was a c20xe 16v vauxhall unit, converted to dry sump and hydraulic clutch, when cold it was fine, when hot progressive clutch slip, back to the guys who built it, re-bled the system, check things over, all OK for the first 40 mins of driving then slipping again. That new clutch lasted a shade over 2200 miles, back again and only thing to do was to have it all apart to have a look, out came the box, clutch worn down to the rivets, all seals were OK and release bearing was still brand new looking, it was thought the clutch had been mismatched with the flywheel, so we changed that for a lightened item (any reason for an upgrade) together with another clutch. This time it was OK but after 200 miles the problem returned, The only thing we hadn't changed was the master cylinder (it was a brand new item) and talking to a chap who runs a c20xe competitively and who also works on a lot of agricultural equipment said "its the MC, I've seen this on a number of items, when its getting hot the m/cylinder seal is preventing the fluid to return which gradually applies pressure which then results in clutch slip" (or something like that, it was 4 years ago). So a new MS was bought and bingo, problem gone..... It may help..