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.
'96 328iC "Marie"
R.I.P 14 November 2013