I'm new to this forum. I've owned a 1988 K100 LT, and a  1988 K75 S. I currently own a 1986 K75 T. Getting ready to replace the tranny. The tranny jumps out of second gear momentarily when accelerating, so rather than having it repaired, or trying to repair it myself, I picked up another one from a 1993 K75 S. Looking forward to being this forum in learning and contributing.

Sounds like it's possibly the well-known loose grubscrew problem. Not a difficult fix when the gearbox is out.

https://www.k100-forum.com/t5030-final-drive-checkup-and-dealing-with-that-floppy-grub-screw

Post 20 for a good pic.