2002 Vmax 200 rpm issue

[ChadG]

2002 OX66 3.1 Vmax. Model VX200TLRA
2002 Basscat Pantera Classic
PTP 2.5"
Prop Yamaha 25M and 26" Trophy
6" Jackplate
WP is 15-20 PSI
4 batteries, Ultrex Trolling motor
Full gas tank
Our issue is that on 1 pass is will run up to 5400 RPMs and hit 66 mph and start to chine. The very next pass is 5000 RPMs and 58MPH. It hit the 5400 RPMs with 3 people in the boat and with 1 person in the boat. Today I was backed in so I took it for pass by myself and it hit 5400 RPMs WOT, I went back to the dock and picked up another person and it dropped to 5000 RPMs WOT. I kicked him out and made another run and was 5000RPMs WOT. I never turned the engine off in the entire process. That was all with the 25M prop. We pulled it out and put the 26" Trophy on and it ran 5000 RPMS WOT. 95% of the time it is at 5000 rpms WOT.
We just got it back from the mechanic. All the linkages are working as they are supposed to, the tach checks out and they put it on the dyno and it rated a 4 or whatever dyno speak is. Mechanics answer was to drop to 23" prop.

We started the setup with the engine at 4.5" and went up to 3.5" and slowly worked up the 2.5" the entire time max rpm was 5000 rpm and sometimes run up to 5400 rpms. All this was with the 25" Yamaha M prop.

This is a new to us boat that had sat basically idle for 2 years. We replaced to 2 main fuel filters and 2 oil filters. Has a new water pump, O2 sensor was fine according to the mechanic.

At loss any and all help is greatly appreciated.

[ChadG]

New o2 sensor has fixed it