Low power under load

I have an '87 50pro that has been just about completely restored. I've redone the carbs, put new coils on, new plugs, new thermostat, fuel pump, water pump, even a tilt/trim motor. Basically I put in more $$ than this things worth. It starts fine, idles fine, but it just won't run. Compression is there w/ 115-120psi each cylinder. Sometimes it will let me go WOT to about 4000 rpms then it will just back down to about 2000 rpms while under load. I'll back it down, put it neutral & it will rev as high as I want. On muffs, I don't have this problem. I seafoamed it 2x & researched this forum & others. Any ideas???

[boats.net]

That is usually a fuel problem, fuel restriction in fuel filter, fuel lines or pickup tube in gas tank, try running on a different gas tank and hose.

Brand new hose & I have 2 fuel tanks. Tried running on both. Fuel filter has been cleaned.