I have an '86 Mark Twain 210SF with a 4.3L Mercruiser. After about 30
minutes running, it loses power and when you slow down, it surges. Let it
cool and runs fine. I have a clear filter at the carb and it shows full of
fuel so I suspect ignition.

I doubt that the dist cap, plug wires, coil or module have ever been
changed. Any thoughts on what to try first?