The last few flights on my AF-4500 I have started getting fluctuating RPM readings where the value rapidly goes up from where it should be (I have a LightSpeed on one of the mags with a digital display) by 500-700 RPMs and then back down again.

On take-off the fluctuations (up and back again) are most pronounced with the cruise being smaller fluctuations.

It has worked fine for over a year.

I assume if it was a bad ground it would be fluctuating between zero and the proper value? In this case it is fluctuating upwards, not down.