Scottie - do you have any excessive radial play between the clutch centre and the basket [like big end play] ? This could cause creep between the plain and the friction plates, due to the forward 'pull' exerted by the engine and induce slippage. Other than that, is there any end float at the clutch centre / mainshaft [should be none], or is the clutch operating mechanism 'bottomed out' somewhere when there is slack in the cable - I have no experience of the scissor clutch, but I am thinking along the lines of when a clutch operating arm can go hard against the inside of an outer gearbox cover and cause slip, even with slack in the cable?
B.W.