Need help , please!!! YAMAHA 250, OX66, 1998

PROLINE 2950 w/ twin Yamaha 250 , OX66 1998 starboard engine fluctuating and don't go over 2900 rpm on WOT, but in neutral it show up to 5000rpm, I did swap every possible parts from port engine, went to 2 different Yamaha dealers and they don't have any clue what to do and this is going for 7 weeks.
Please any advice appreciated.

Have you swapped out the entire fuel system (incl VST, LP pumps etc) and injectors? Sounds like a typical VST or LP pump issue at a first guess, but the Yamaha mechs should have been all over that first up. The other similar issue we had on an HPDI a few years back came back to a bad earth. There a bunch of small earth wires on the side of most Yamaha blocks. I would spend an hour undoing each one, cleaning them, and tightening them back up. Quick squirt of touch up paint afterwards to protect them again. Fuel is more likely, but the Yamaha mechs and presumably been all through that already. The other quick check would be to run it off a tote tank in case you have fuel delivery issues in the lines or tank.

Start with the basics,compression test,open air gap spark test,then on to fuel diagnostics

degraded liner in fuel hose? the liner in a short fuel hoses in my engine was separating from the inside of house. It collapsed as RPMs/fuel flow increased., choking fuel flow. Would start and run like a champ, spool up to 5400 rpm, then in a few seconds RPMs would start dropping and level off at about 4500rpm. Replaced line, all is well.