Electronic tachometers give erratic readings when the base voltage is low, usually caused by a loose battery connection. The worse situation for low voltage is at idle when the alternator output is at it’s lowest. You may just need a new battery or a good charging.
If it's not the battery:
Most likely - Intermittent power or poor ground connection
Next likely - Tachometer internal damping problems
Check for a loose or corroded electrical connection at the battery, the fuse, and any in-line connector. Check the two battery connections first, next the connector at the instrument cluster, then at the fuse, and lastly at the wiring harness connections.
Next, since the bike is running OK, the RPM sensor gap, the sensor itself, and wiring back to the computer are not the problem. So you need to check the connectors between the computer and the tach.
Finally, try to borrow a tach to verify that it’s actually the problem, before buying a replacement.
Of course you could bought the wrong tach. The tachs are different for the P8 and 1.6M computers. Internally, the tachs have a different circuit board for the respective computers. Externally, they appear the same (I think.)
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Linear Mode
