Have you pulled the fuel tank pick up tube and examined the lower most section?
Have you checked and/or replaced the anti-siphon valve during any maintenance?
Is the fuel filter of the correct micron rating, and has it been replaced?
Does your carburetor incorporate a small filter unit... if so, has this been replaced?
If not fuel related, check to see if your ignition system is offering the correct progressive spark lead.
See your OEM manual for this spec.
While strobing your timing marks, increase RPM to approx 3,200rpm, and post back with the number.
If the ignition system is not producing the correct TA (total advance), it will greatly affect performance.
Look to see if the engine is equipped with a rev-limiter. If so, and if malfunctioning, it may cause this.
Is your hull clean? Marine growth can also cause this.
Have you changed propellers prior to this?
Any other changes made to the boat prior to this?
.