Start looking for another tach.
These things have two things that go wrong with them internally.
1) the needle is on a "jewel movement". It wears out then the thing is junk 2) there is a printed card in the tach. It cannot be replaced. If it fails, the tach is for display purposes only.
Your's is showing the symtoms. Look for a spare tach now is what I would do?
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