Admittedly nothing is impossible but surging is more like a fuel
problem to me than an ignition problem. Ignition usueally works or
doesn't work. Heat related ignition problems are usually a coil,
pickup, or module failure that just quit when they get hot and then
start working again when they cool down. Check for spark at one of the
plugs immediately after it dies. Surging is often a fuel delivery
issue. Maybe you have a vapor lock problem? The fuel is boiling in
the line or carb. Have you had any changes related to cooling? Is it
running hotter than it had in the past?

wrote:
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?