Possibe electrical problems or is a sensor please help!!!

New to this forum but very informative and hoping some of you guru's on here can help me. In may of this year i purchased a 1998 fourwinns 225 sundowner with a 5.0gi volvo penta and cobra sx outdrive, the engine now has 560 hrs on it. The boat has worked like a charm all summer long but last weekend the problems began, at first when the speedometer would reach 40 mph it would start to sputter and loose ground fast then all the gauges would go wacky and the engine would start making a smell like rotten eggs, then once i shut the boat off it would re-start and run fine at idle as soon as i put it in drive forward it would make the gauges go crazy again, so neutralized the throttle soon as i would rev it up the gauges would act again. I have traced wires to no end and cleaned all connections and added di-electric grease to all contacts checked all fuses and both relays at the fuel pump now im stumped. Any feed back would be greatly appreciated.