When I get above 3500 RPM and I really getting on it, my tach needle 'wiggles' back and forth so much so, I don't know my RPM. I just split the difference…. and shift. I've had this happen before, like 40 years ago, and all it was then was just a loose tachometer cable where it attached to the transmission. (Triumph). Thoughts?

 

Shift by feel and sound, ignore the tach IMHO. No idea why it would shake so bad, I hardly ever look at mine.