I've had this motor for around 15 years and it has been virtually bulletproof.  Last year was the first ever issue, which ended up being a $20 solenoid.

This year, I'm having issues with the motor sputtering or chugging at high RPM, especially when warm.  It basically will not achieve higher end RPM's and will chug along revving up and down and not maintaining a consistent powerband.

When the issue first started, I did the easy stuff:  fuel filter and spark plugs.  That didn't work.  I started to worry bad gas or moisture in the fuel.  I completely emptied the tank and put all fresh fuel in.  It ran great for two outings.  Then, the last outing, after it was warmed up and had run fine at 3/4 throttle for some time, the issue returned.

I'm at a loss and not sure where to start.  Is this a fuel delivery issue (fuel pump) or an electrical one?  

EDIT:  I know there were some oil injection issues with this vintage.  However, the prior owner had the oil injection system removed, so I've been running 50:1 mix since owning the boat.