What's with the tachometer?

Tach quit working, except sometimes it fluctuates wildly. Cigarette filter under the center in the amplifier makes it work, for awhile. Then it stops again. New filter gets it going again. What is happening here?
How can I fix it permanently? Car is '81 300SD
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Buy a new "tach amp" - the cap with a new circuit board inside. But be forewarned that there is no "permanent fix" other than unplugging the tach amp and forgetting about the tach.
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snipped-for-privacy@NoHamOrSpamcomcast.net says...

Of course, the tach serves no real function in a passenger car most of the time. But I hate things that exist but don't work.
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