Im having the same problem i fitted a heavy duty clutch to my rs turbo cab and im having the same problem it slips when the boost kicks in.One thing i never did is i didnt fit the spacer plate bewteen the engine and gearbox i dont no if this has any thing to do with it slipping have u fitted it on yours?,I no the clutch pedal can cause problems there is a self adjuster on the pedal and if the teeth are worn it could make your clutch slip u can buy one off ebay its worth a try mate