Surging Mercury Outboard, 2014 60 hp EFI 4 stroke. Alumacraft boat with EPA compliant fuel tank and fuel fill. Starts great and runs great for about 20 minutes on plane. Gradually loses power then begins to surge. Eventually will stall out. Replaced one fuel filter and ran it off of a portable fuel tank to check for venting issue. I has run great for 3 years and the only thing I have changed is I have added a 4 blade prop. Don't see how that would make a difference.