I agree with Bob that the tach board can cause intermittent tach operation though the way your needle is acting it almost seems that the tach movement may have a mechanical or electrical problem.When you pull the instrument cluster to check and clean the tach board contacts you may want to go a bit further and disassemble the cluster further and pop out the tach mechanism. It's easy to remove. See if there's something binding in the mechanism or maybe the tach clockspring ( if there is one ) may may broken or detached. Typically you can move the tach pointer in the full range of motion with as little effort as blowing on the needle.
This would also be a good time to clean up the instrument faces and the back of the lens and to check or replace any burned out or blackened instrument bulbs.
This would also be a good time to clean up the instrument faces and the back of the lens and to check or replace any burned out or blackened instrument bulbs.
