No top end

[BayBoy]

Hey fellow boaters. Got another issue. I have a 1999 Maxum 3200 SCR w/ 5.7 2lb Bravo II's. Engines are running great and, while in neutral, will rev right up to 4800 rpm with no problem. Note: I do not do this on a regular basis, just trying to get to the bottom of the problem. Drives engage smoothly and I can tool around at displacement speed with no issue. When I drop the hammer it struggles to get up on plane even with the drives and trim tabs all the way down. I can barely hit 28 knots and the engines max out at 3600 to 3800 rpm (used come right up and hit 36 knots @ 4200 rpm with no problem). While I have had the drives off to replace the trim limit switch and indicator along with bellows, the cases have never been split. I did start using the synthetic gear oil several years ago but it supposed to be compatible.

Any ideas?????