I assume you are running a carburated Mercruiser with their Thunderbolt
solid state ignition. I had a similar problem with one of the 4.3L V6
motors in my 'Dancer. I found a clogged vent had caused some corrosion on
the trigger rotor inside the distributor that was intermittently causing
extra pulses, causing high readings on the tachometer. I cleaned up the
interior of the distributor and polished the rotor. That took care of my
tach problem and also smoothed out the idle on that motor.

Hope this helps. Good luck.