Are you actualy dropping rpms? If so, next time it does it, squeeze the primer bulb and see if it recovers. May have a bad squeeze bulb causing bubbles in your fuel supply. Is Your's Quicksilver OEM? Heard after market bulbs are unreliable. Could also have flakes of crud coming off your fuel line. I had it happen a couple of years on a 2002 90. Had crescent shaped tan stuff all through the line. Turned out to be the line between the tank and engine scaling. Wouldn't hurt to replace all your fuel line rubber and internal fuel filter too. with your age.
If you aren't actually dropping them, clean up the wires going to your tach. Could be as simple as taking a 11/32 (green as I recall) or a blue ⅜ nut driver and back the nuts off then run them on a few times and snug up.......shine up the contacts. The engine end should be ok as they are pretty well protected by Merc.
