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So the RPM reading on my previously beloved EDM-900 went nuts last week. It works ok at startup, assuming the plane has been sitting for a while. Then at some point in cruise flight, the reading starts fluctuating wildly (it's definitely the monitor system - not the engine or governor). Then it stops reading outright and indicates the big red X. I had two different mechanics look at it briefly on the rather long, 8 leg trip I took last week but they couldn't do much. The rpm sensor that goes in the inside bottom vent plug hole of my right mag (Bendix) is very hard to access and may require pulling the mag to check it fully. It looked ok with a mirror and feels secure. One of my stops was at Oshkosh, where I got to ask Joe Polizzotto, the CEO of JPI, about it at their exhibit booth. He said it wasn't a common issue that he was aware of, but happened to have a new sensor on hand and gave it to me. He also said that it could be caused by the magnets in an old mag getting weak - I'm not sure I totally get this - could the problem really be intrinsic to the mag? My mags did have their 500hr service 6 months ago. Questions: 1. Anyone with similar experiences out there that got addressed? 2. Any other guidance to give my avionics shop when I take it back to them to swap the sensor? I will be very frustrated if the sensor gets swapped and I still have the problem- it can take up to an hour to show up in flight, so I may be back home before I know if the fix worked.