Could be a bad connection, or perhaps a problem with the alternator...The tach gets its signal from the alternator windings. So, if the alternator is messed up, the tach could act erractically.
The simplest thing to do would borrow another tach and plug it into the connector on your control box...I realize it might be tough to find a donor tach, but possible.
Those rubber amphenol connectors can be troublesome also. Pull the tach plug off and look inside, the three pins should be straight, and make sure none are pushed back in the connector. Try to peek at the socket end of the connector on the control box as well, perhaps one of the sockets has pushed back in its connector.
Please do not cut the leads or attempt to lube the rubber connector with grease or WD40, this could create a bit of a short. Soak the two halves of the connector with rubbing alcohol before reassembly, the alcohol will lube the rubber and evaporate quickly.
I suppose I should have began my reply with checking the "pulse setting" on the tach. It may be very hard to see buried behind the dash, but the OMC tachs should be set to position "6" for your engine.
Like Frank says, check simple things first, like battery connections/cables. You could try wobbling the rubber tach connector while the tach is misbehaving to see if it straightens out. How is the tach misbehaving?