I've been having this exact problem with my spec 6 puk, i bled the clutch system wich temp fixed it, i then unbolted the slave an tilted it up so the nippple was at the very top, a lot of air trapped in the system, then ran fresh fluid thru, fixed it for a couple days, so it may not be a problem with the clutch, but the slave cant take the added pressure, I've got a slave on order from td, ill let ya know in a couple days if it sorts it , easier than getting gearbox an clutch out again, also it drove lovely when i had bled the system