OX66 loses power above 5200 RPMs

[TommyOX66]

Thanks in advance for any help. I have a 2005 OX66 200. The motor cold starts fine and will run WOT for the first 10-15 minutes like it's brand new. After it warms up it will not run hotter than 5100, has trouble starting, and idles rough. Local dealer said to run it in water at idle and pull plug wires off to see if a cylinder acts differently. I pulled off all and the only one that runs funky is the top right (don't know the number) looking at the back of the motor. Any ideas, maybe a gone seal that breaks when it warms up? I was told it was a CO2 sensor, local dealer said he never had one go bad and to clean it. Tried that also. I'm lost.

This may also be helpful: The CO2 sensor bolt keeps backing out for some reason. When I pull off the cover there is black liquid residue around the sensor.

[99yam40]

I am fairly sure there is no CO2 sensor on any motor

I would monitor fuel pressure when the problem occurs to see if that stays in spec

[TommyOX66]

Edit: O2 Sensor