I think it would be an idea to swap the relay with one of your k75 models, especially if the part numbers are the same (probably check the wiring diagram though)
I recently swapped flasher relays from an '84 k100 with a '90 k75, they both worked on either bike, even though the part numbers were different.
If it's faulty and you want to replace it anyway, might as well open it up, in the hope that there might be some obvious, easy to fix, problem.
Just curious, is there a small blip from the tacho needle when you turn on the right or left turn signals? There shouldn't be.
For some reason, when you fix some types of flasher whack, the symptoms remain for a few days or uses, until it goes back to normal.