Could be the tach, but it could also be a poor connection. If it's the small tach, you could check power and ground, both at the rear of the tach and by the fuse box + wherever it connects to ground. If the tach light is affected too, my guess is it's caused by a poor contact.

Intermittant errors like this are a pain. If the connections seem good, it would be easiest to try with another, known-to-be-working, tach if you manage to borrow one. If the problem persists, then the problem lays elsewhere.

Erling.
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