98 Merc 30hp ELPTO Voltage Problem

[428Scooter]

Recent problem. Owned boat / motor (Smokercraft / Mercury 30hp ELPTO) for four years with no problems. Serviced annually. Recently when driving the boat I noticed the voltmeter and tachometer indicators bouncing back and forth across the scale on the gauges at high speed (4800 rpm). All seems to stabilize at trolling speed (900 rpm). Battery properly charged and cable connetions tight. Checked connections to gauges under dash and all seems o/k. When this happens my fish finder also acts up - reads inproper water depth, etc. Took it into shop and marine mechanic could not identify the problem.

Thanks Dave
Peterborough