Do you still have the stock relay board?
A bouncy tach can be caused by dirty or lose connections or broken solder joints on the relay board.
I had the same issue when i bought my 914-4 12 years ago. Back then you could still buy brand new relay boards, which is what i did and the bouncy tach issue went away. I'm guessing my old board had tired solder joints where they go through the relay board.
There's a few nice write-ups here on how to refurbish your relay board and redo the soldering.