Motor revs up and down but stalls when you try to hold a consistent RPM

[rhodesmotor9.9]

I have a Yamaha 9.9hp 4 stroke. We just started it for the first time this season after being winterized and it started right up. The motor held the idle speed in neutral or in gear just fine but as soon as we increased the RPMs, the motor bogged and stalls out. The strange thing that I cant figure out is that the motor will hold the high RPMs just fine if you constantly fluctuate the speed up and down (like revving a race car) but as soon as you try to hold a consistent, high RPM, the motor will stall out.

Any idea what is causing this? We bypassed the fuel filter and it still not holding a constant speed without dying.

Thanks in advance!