1991 Commodore 360 Tachometer issues

I have a Commodore 360 that has twin 454/7.4 carbureted Mercruisers and V-Drives....the boat has VDO brand engine instruments and I'm trying to determine whether the problem I am seeing is related to the tach or not. With engines running, the starboard tach indication seems somewhat more reliable (most of the time)....cruising at about 8 knots the starboard engine indicates about 1500 rpm and the port tach will indicate 2000-3000 rpm much of the time, though by the sound of the engines, I can tell they are running pretty much the same RPM. Tapping on the port side tach or the instrument panel next to the instrument the RPM needle will jump around...2500, 3000, 4000, 1000 RPM, etc....considering replacing both tachs but would like to confirm that is the likely problem before spending the $150 (or more)....there is also a sync gauge between the two tachs that never moves off of zero...even when the engines are obviously at big RPM differences. I am also assuming (as I've not taken the instrument panel loose yet) that the tachs are electronically driven off of the ignition coil as opposed to mechanical (from a cable).

Has anyone else had similar issues on an older Regal like this that could offer some experiential insight? This is a 30 year old vessel that has averaged about 40 operational hours per year but probably sat idle for some extended time before it was purchased by the owner I bought it from about five months ago....he had owned it for less than 18 months....only motoring out to a cove from the marina and back to the slip....so it didn't see much use even then.

Thanks in advance - JJ