Usually when the slave cylinder fails it keeps the clutch from disengaging, it doesn't cause it to slip. To cause a slip the slave would have keep the clutch from releasing fully.
Have you noticed any oil leaks coming from the area where the transmission mates to the engine? A leaking engine rear main seal can get oil onto the clutch disk and cause a bit time slippage.
2006 Z4 3.0si Sport Package (But I'll still hang with my "homies" in the E36 forum)