Took the boat out of DP for some fishin' today, but this is not a fish report. My boat is a Skip 28 with twin Volvo 350's and 280SP outdrives. I ran about 5 miles W to look for paddies as I was headed S to the Barn Kelp. 1/2 way there I crossed paths with a large yacht so I slowed from my cruise rpm of 3400 down to about 2000, reset the trim tabs to bow up, crossed the wakes then went to get back on plane. In accelerating back onto plane again the starboard engine / outdrive wasn't performing like normal and seemed to be cavitating with variable rpms. But the port engine and outdrive ran flawless all day. So I did just about everything could to eliminate what seemed to be cavitation. In neutral, the engine would run fine. In gear, the engine would be inconsistent in rpms and wouldn't run like the port engine. So I had to cruise at about 1200 rpms over towards the San Mateo Kelp. With a mile to go, I tried it again, and it ran as it should have, back up to 3400 rpms and all was fine. And after spending hours there fishing on the hook, we decided to head on back. Both engines ran perfect for the 8 mile return trip to DP harbor, then upon backing down to 1200 - 1000 rpms on entering the harbor, the starboard engine started again, then dropped rpms and shut down. I started it back up and it ran with inconsistent rpms again, and I put it back in the temp slip I have been renting for the time being. About 6 weeks ago I pulled the engines to pull my fuel tanks and put in a new main tank, then reinstalled the engines. I've used the boat for a couple similar trips between today and the new fuel tank work without issue, and that's why this is puzzling. I was expecting that if there was a problem, it would have shown up previously. Anyone ever experienced something like this and understand what's going on here, or have anything to offer please? . . .