I don't know if this helps, but I've just solved a similar issue. I recently purchased a 88 k75s. The tach would get stuck if the bike was idling. If I revved the motor the tach would get unstuck and go up to about 5-6000 rpm (wherever I probably revved the engine up to). When I left off the throttle quickly it would stay at 5000rpm.
Last week, with a poor forecast on the horizon, I took off the instrument panel and went to investigate further. I figured I had nothing to lose so I dove right in. I carefully used two forks to pry the needle off of tach shaft, unscrewed the black faceplate, removed the gpi (gear position indicator), and then removed the tach assembly. It turned out the round magnet guide that the tach shaft spins on was a bit corroded. I carefully took apart the tach assembly (being very careful of the copper coils) and cleaned the guide magnet. It was significantly better, but it was still jamming a bit. I then used a small piece of casing from 22gauge wire to raise the shaft a bit. I reassembled the tach and it was spinning smoothly. To test I hooked up a 9v to the two pins on the tach assembly. You want the positive lead attached to the left pin, negative to the right. If it works, great, if not try hooking the positive lead to the right and negative to the left. If that works, then you installed the magnet upside down - fix that.
Before you reassemble the tach, hook the instrument cluser to the bike as is, start it up and measure the voltage across the two connector for the tach on the tach circuit board . You should be getting voltage around 9v when you rev it, it should get higher like 10-12v. If that's good, Reassemble and hopefully you are good to go! I'm kicking myself for not taking pics. Anyways I just typed this on my phone, so let me know if/where you might need clarification
UPDATE: everything still appears to be working. The tach is a little bouncy but is still accurate.
