I'll share my unexpected "fix" for a dragging/lagging tach on my 1978 GL1000. It had started lagging badly as they have a tendency to do so I began researching places and ways to fix it. I had looked at the overseas repair place and also at an in-country shop (very expensive) and had not decided what I was going to do.
In the meantime I took a day trip which had some extensive sections of pavement with very bad expansion joint bumps, to the point of it being a bone-jarring ride for quite some time.
I didn't think anything about it, and when I got to my destination and was going to show my brother (who is always interested in things mechanical) the tachometer issue, when I started up and went to demonstrate, the tach was now working normally!
I can only speculate as to why/how this happened, and the only difference between this and all of the other rides I had taken without any changes to the tach situation was the amount of bone-jarring bumps that this particular ride contained.
Even though this happened over a year ago, the tach has worked perfectly since then.