Clutch seems to jump on engagement-91 325xi

I have a 91 325xi (5 speed)with 40k. Sometimes, maybe every 10th time, when releasing the clutch the drivetrain engages, then it feels like something jumps (momentarly disengages) and then it engages again. Didn't happen for the first

37K. Had dealer check clutch, but thought everything ok. Any help will be greatly appreciated