This happened about a month ago also, but I chalked it up to possibly an ignition problem and after I finally got the boat running great, it happened again this evening. My "ignition problem" turned out to be a broken spark plug.
Anyway, I was out running around a bit after supper tonight, went for a 10 mile ride, part of which was in pretty good chop. Coming up river towards home (in smooth water), I had been cruising for about 10 minutes at 3200 RPM and SCREEEECH, hideous sound and I lost propulsion for about 2 seconds. Pulled back on the throttle, it went away and it ran just fine. To my untrained ears, it almost sounded like a spun bearing or something to the effect. It wasn't long enough to determine if the engine had actually cut out or not, but I don't think so, or if it came from the transmission or even the propeller hub. It was pretty unsettling though. Temp and oil pressure were fine, clean fuel, new plugs and even a new ignition. I changed the outdrive oil not 2 weeks ago and also changed out the tranny oil maybe 2 months back. Otherwise, it has been running just great. Have had no signs of the tranny slipping out of the hole either, but I'm easy on it.
If it's any clue, it seems to happen when I am running a constant RPM for a while? 22c, 454/330 TRS w/Merctrans
Open to any ideas?