Not sure if I have had the same issue as the OP but I have described mine as "hanging". Has been discussed a couple times, if not here then over on the T-rat site.
For example mine will sometimes "hang" at 1,000 RPM when the motor is shut off instead of returning to zero. It will then read high by that amount the next time the motor is cranked, now actually idling at 1,000 RPM but displaying 2,000 RPM. Can then "hang" at 2,000 RPM when shut off. Then reading high by 2,000 RPM next time, idling at 1,000 RPM but reading 3,000. It will zero itself at shutoff whenever it feels like it.
When it first began I found I could turn the key on and cycle the thumb kill switch a few times to zero the tach display but that no longer works for me.
