1987 90 HP running poorly

Hello all.
I have an older (1987) low hour 90 HP 2 stroke. Motor ran fine for first 20 or 30 hours since I acquired it. Then, while running at 4500 RPM in bouncy conditions it began reducing RPM until it finally quit running. It would always re-start but would not run well until it had sat a few minutes. Then, as long as I ran at 3500 or less RPM would run fine. However, it now quits after hitting a few boat wakes. Then, I ran it for several hours sitting at dock at 2000 RPM with no problem. I have replaced fuel pump, water pump impeller, thermostat.

I am now replacing 26 gal fiberglass fuel tank in 17 Whaler with a new plastic one. I am also replacing fuel lines and fuel filters as well. I suspect my problem is ethanol and I hope the new tank and fuel lines will cure the problem.
However, why will it run ok at lower RPMS if it is ethanol related?
What else could the problem be?
Any help or suggestions will be appreciated.

Best regards,

Larry 7777
Key Largo

[boats.net]

Probably need to remove and clean carbs. It has a fuel restriction and not getting enough gas to stay running.

Thanks. I forgot to mention I had carbs cleaned and new carb kit installed for all 3 carbs which I coincindentally bought from you. (best prices on Yamaha parts anywhere) but maybe they need cleaning again?

[boats.net]

Get you a portable gas tank and hose, connect hose to engine fuel filter and run motor. That will tell you if problem is in boat fuel system or on the engine.