I’m not sure if it’s of help, but a while back, my rev. Counter showed 8000 rpm at tickover, and was erratic. That was a Mercury Rev counter, with a Mariner outboard, so obviously a very different set up. I eventually found some help on a Faria gauge website.
Even though a different make, it suggested moving the settings on the rear of the Rev counter, from its correct setting for number of cylinders, to a “ wrong setting”, then back to correct setting. This seemed to reset the gauge, and I never had a problem afterwards.
It’s a simple thing to try first.