1999 200 ox66 RPM loss

I have owned this motor for 3 years. Until the last 3-4 months, it has consistantly ran 5700rpms at 60 mph. Now it will only run 55mph at 5200rpms the only way that I can get more RPMs is to overtrim the motor. I thought that it would be a low pressure fuel pump issue, so I replaced them w/ pumps from iboats, and cleaned my fuel filter w/ high pressure air. This had no effect on the RPMs.

Any Ideas??

Mine lost top end rpm's when my O2 sensor was gunked up. I lost more than 500, but it is free to check it out.

I forgot to add that I took O2 sensor and dumbell off and soaked in seafoam overnight about a month ago. It had no effect either. I am going to search for the post regarding testing the O2 sensor I'll do that next.