I have this 225 merc that is new to me. I bought it from a pal that owned it for years. I put it on my boat this early spring(still winter actually) and it ran fine. I have been useing 6 gal plastic gas tanks,several on each trip. What a thirsty motor. Last trip out it was running rough at low rpm as I was heading to the dock. It was skipping and backfired a few times then stalled. Started right back up and took it for a quick run to try to clear out what ever was in it. It ran fine no miss. Came back down to low rpm in gear. Did the same thing but did not stall. Was running rough very bumpy,surging and just barley running.
I removed the fuel filter and it looked OK. The gas was nice and clear with no signs of water. I replaced the filter and it seemed to run and idle fine. As soon a took it out and slowed to headway speed it started running rough again. The engine only has 25 hours since the last fuel filter and spark plugs were changed and the compression at that time was 115 to 120 range. Runs super strong above low speeds nice and smooth. Seems like a lean cyl or bad coil type of thing. Anyone been down this road before????
Thanks!

Drain the Vapor Seperator Tank and check for water. Check the plugs, are they all burning the same? How is the WOT rpm, is it not taching up as high it used to?