Wandering Tach

The starboard tach on my 1993 370 Carver AC jumps all over the place at RPM's above approximately 1000 RPM. It does this throughout the normal operating range, jumping as high as 5000 and dropping back to near normal. Replaced the tach last year and this one does exactly the same thing. It reads accurately without any jumping around below 1000 RPM. A friend three slips away reports his 1992 Carver 36 has the same problem. Can anyone help with diagnosis? Engines are 454 Crusaders.

Capt. J,
Thank you for the response. I should have mentioned that the boat only has 400 hrs. on it since it was in storage for 6 years before I bought it in 2012. Would that be enough time for the sender to get off? I went to write a service order to get it fixed yesterday and the service manager suggested I contact Carver to see if there was maybe a service bulletin on it when I mentioned a neighboring boat having the same problem. That's why I tried this site too. I was really surprised when he mentioned that he thought there was a sender. I figured that by 1993 the boating industry would have caught up with the automotive industry and gone all electronic on tachs (i.e., no senders).