Is it lean popping at all? Is it sneezing at you, like it has a bad
cold?

You could have a host of problems here, especially if the engine is 15
years old and you don't know the history of it. I'd be curious to know
if you solved the problem. Often a 2 stroke engine will run just fine
with a host of problems at mid or full throttle. But when you slow down
to idle, that's when they show up. And that's the most important time,
as that is when you dock.

There is an entire diagnostic procedure for these little Yamie's. And
the best place to get one is, you guessed it, your service manual. By
all means, try the "gumout" treatment. But I doubt it will solve the
problem. But the nice thing is, these little Yamaha's are easy to work
on with the right tools and a service manual.