[spuggy]

Bad power or bad ground could possibly cause the symptoms.

Many people report the tach erratically reading high when the voltage regulator on the alternator goes south. Doesn't always happen; my tach was steady as a rock, but after cleaning huge fuzzballs off the negative battery terminal for a couple of weeks running, I checked voltage - and found the VR was kicking out 18V at idle and boiling the battery..

My speedo seemed to have developed a "tremor" after the alternator was replaced; would read one speed for a few seconds, kick 4-5 MPH high for a few seconds, drop 4-5 MPH low then repeat. Did that for a couple of years before it suddenly died completely.

Both the speedo and the tach have electrolytic capacitors in them, which do not have an infinite service life to begin with (they often swell/leak when they break down, but can also just stop working with no external clues). Added to which, the original parts are rated at 15V, stuffing 18V into them does not make them work longer/better.