As weird as it sounds, I have a had a clutch that would slip like that after the car set for a few weeks. The rear main seal had enough of a leak that oil would run down the transmission input shaft and saturate the clutch just enough. Whenever the car was first driven after sitting, the clutch would slip a lot. After a day or so the clutch would behave normally again until I would let it sit again. Check the rear main.
