40 HP Evinrude Outboard runs bad forward, fine neutral/reverse

Hello all,
This is my first post to this forum, or any forum for that matter.
I have a 40 HP Evinrude outboard, i believe it is a 1992. I'm not sure off the top of my head, and i'm at work so i can't check at the moment.
The motor is on a 1994 24' Crestliner pontoon.
The motor has a VRO fuel pump. The oil intake has been deactivated, adequately sealed and wiring disconnected. I am pre-mixing 50:1
The issue i am having is that the motor often has difficulty starting, needing to be put in advance throttle and choked to get started. Once started, it often will eventually sputter and die at an idle. PUMPING THE PRIMER BULB DOES NOT HELP OR HURT

While in the water, when put in gear and fully throttled up, the engine will only run forward to what seems to be 50% full rpm, like it is only at half throttle. If throttled up in neutral or reverse, the engine will scream no problem. HOWEVER, when on the trailer with the muffs on, the engine will run fine in forward, reverse, or neutral.
Here is the work that i have had done/done myself, to avoid repetitiveness:
Rebuilt current VRO fuel pump with kit purchased online
Replaced all fuel line, primer bulb,fuel filter, and connections from fuel pump to tank
Removed tank and cleaned
Spark plugs have been frequently changed
Both cylinders firing when checked while running

I have had it to a marine mechanic twice and he said "It runs fine", but without running it in the water. I realize the mechanic clearly isn't very thorough. I guess thats it for now, what do you think?

The motor is not new to the boat, when i purchased the boat it had that motor on it. It has moments where it will spring to life and run fantastic, usually upon start up if it's going to happen at all. But within a few minutes, it will drop off to about half speed every time. I agree, it is a bit more boat than the motor would prefer, but i'm just looking to get around the lake and don't want to get a new motor. I don't have compression numbers, i haven't run a compression test but i will get a gauge and do that. I don't have a ton of boat motor experience, but i'm learning what I can. Why would the motor get high rpms in reverse but not forward?

Overheating ????------Have you changed the impeller on it.--------First thing to do with a new to you motor !!

Do a cylinder drop test on it when it drops off to half speed. That may tell you something.

Thanks for the input everyone. I have not changed the impeller, it is cycling water fine out of the tell-tale. I should probably go ahead and do it just because, i'm sure it's been awhile.
Update: On the advice of a work associate, I bought some sea-foam fogging oil, and (per the instructions) loaded carbs up with it through the air intake, giving more throttle and then allowing to hot soak for about 10 minutes. It ran very good for the duration of the trip that day, but the motor was only running ~30 minutes total for the whole day.
Still haven't tried the compression test