There is one more possibility that is a really rare occurrence

but it happened to me. I had taken the clutch apart

several times and could not find anything wrong with it

but if I would partially engage the clutch it would grab

and creep, sometimes pulling pretty hard even if I floored

the clutch, until I yanked it out of gear, then it was normal

until partially engaged again. Real pain in the backside

and I asked LOTS of people what it could be, with no other

answers than "it is a hydraulic problem" (I had all new hydraulic

components so I was pretty sure it wasn't.)

Turned out I was using a reconditioned BMW clutch disk

which has a metal button just inboard of the friction surface

and is supposed to signal you (I guess by making noise) that

you need to replace the clutch plate. I put in a clutch disk

that didn't have this button and the problem has never

recurred. The button is the head of a rivet, so using

your trusty Dremel Moto-Tool and your #409 abrasive

cutoff disks you can remove this rivet and probably

accomplish the same thing. My facings were worn pretty

bad by the time I figured this out. I think what it did was

to cock the disk on the spline so it couldn't move as

freely in the axial direction.