It is the clutch itself. I would say it is the clutch disc that has uneven grip around it's perimeter when hot, probably due to the friction material used since BMW went to non-asbestos. Here are 2 more links on the problem:
http://digest.net/bmw/archive/v5/msg01583.html http://66.102.7.104/search?q=cache:2...&hl=en&start=5