Originally Posted by slow3v
Could be rear main seal leaking on the clutch. You saying it happens more when the car is warm makes me think that.
My old Golfs rear main seal went and doused the clutch in oil. Gripped fine cold, drove for a few miles and it got awful. You'll need a new clutch if that is the case.
It slips worse after the car is warmed up because the clutch has been slipping during that warm-up time, heating up from slipping, then slips really bad until it can't be ignored. the driver notices "It slips after it warms up"