I have the JPI EDM 900 all-in-one.  Everything else is working fine, so is the RPM most of the time.  Last flight, after about 3.5 hours it started jumping around.  Always to a higher RPM than actual, then back to normal, high enough to trigger the engine warning.

Mike,

 

Whether the pickup is a Hall effect sensor on the mag, crankshaft sensor or the old Ford tach cable converter, the output is a series of pulses, usually 1 per revolution. In the case of the LASAR mags or the Light Speed Electronic ignition, there are two pulses per rev.  It would seem that the only way to get erratic higher RPM's is a loose connection that, when subjected to vibration, appears as more pulses to the computer or a circuitry problem.

 

Let's see, 2500 rpm is about 41.7 rps or 24 milliseconds/revolution, plenty of time for havoc.