I have a 2004 60 hp Yamaha 2 stroke that has been running fine up until last fall. After a day of fishing and pulling the grandkids around on a tube it started bogging down when I throttled up. At idle it runs fine, above 2000 rpms it runs fine, drop under 2000 and she loads up and drops 800-1000 rpms before dying. Throttle forward from idle and it bogs down and shudders for several seconds before kicking in and running. Mechanic thought it was the impeller, replaced it and still have the problem. I run Sea Foam in every tank, I'm thinking this is a carb problem. Any ideas?