I apologize if this issue is somewhere else in the forum.
My pontoon has the 2006 Mercury 40HP. I haven't used the boat much over the last few years, but each year, I've taken it for service at the beginning of each winter for winterization. Here's what has recently (last few weeks) been done:
1. Plugs have been changed
2. Carb kit was competed
3. Old fuel in the tank was removed and replaced with new fuel
4. Fuel line was replaced due to rot
5. New battery
6. Fuel system flushed/cleaned
There's more, but I can't remember right now the rest of the work I've had done.
The issue is that after you've been running around on the water for about 30 minutes, it loses RPMs. It will go up to around 4,000 RPMs and then fall back to about 3,000. When the boat was newer, it would stay at the higher RPMs and cruise at about 21 MPH, but when it drops down to 3,000 RPMs, it's definitely not getting the speeds its capable of.
I think we've narrowed it down to an electrical issue, but not sure if it's the computer (not sure what it's called) or the stator.