Engine idles fine. Comes out of hole fine. But, when running on plane, regardless of rpm, about every 30 seconds or so the rpm will drop about 500 rpm for about 3 to 5 seconds then jump right back up to previous rpm without moving throttle. Does this repeatedly regardless of rpm running. It did this just a couple of times over a days time about a month ago. But I just figured must have been just a shot of bad gas at the time. Has been running good until now. Usually when I go fishing I put just 10 gallons of gas in only tank I use because don't want to leave bunch of gas sitting in tank going stale. I always run ratio of Ringfree at 1 ounce per 10 gallons and Marine Stabil at 1 ounce per 10 gallons. Yesterday I changed out fuel/water separater filter since it's been a while for that. Changed out bulb in fuel line earlier this Spring because original was getting 'hard'. Used Yamaha brand. Every 100 hours I take it to Hydro-Tech for them to go through for tune up, change all filters, water impeller, plugs, and whatever it needs. But, has been a while. Wide open for all advice and things to check. Thanks a lot.