This problem started after (20) several trips where I was idling with the motored trimmed up for long periods as I was fishing back ends of stumpy creeks and real shallow water. Gas is non ethanol 90 oct and I have a water fuel separator canister filter. One day last week It was slow to plane and then missed and lost power then got power back again. I am running Yamaha rig free in it now with 2 oz per gallon and after ten gallons of new gas it is still missing at mid range rpms. I have not changed spark plugs but was going to first as they are three years old. Is this probably a carbon problem as some of my friends say and will spark plugs fix it?