The usual oily residue you refer to in your first post (plugs) ain't usual, seems to me. It's a sign of poor combustion, rich carb setting, too much oil in the gas, incorrect plug gap -- not saying it's some kind of fatal condition, but would see it as potentially related to your running problems. Motor may be stumbling at higher rpms, if it's running too rich. Jets are fixed (and you worked at cleaning them), so think I would look at fuel contamination (water or other), age of gas, and then oil ratio in gas. Have spark on all three when checked (assuming the spark you saw is a good hot spark) -- should be OK, unless ignition is acting up at high rpms (harder to diagnose, but less likely than a fuel related issue.)
Just running a motor can clear issues -- learned from an old outboard mechanic years ago that letting a motor run for an hour is often the only cure needed. Might put in fresh gas and oil, then find an rpm level where the motor runs out smoothly, and just run it there for a half hour or so and enjoy the lake.
not a great fan of sea foam, although has its place. seems like a lot of folks use it as routine maintenance and might create issues. swear by it or at it, not sure.
[edit. assume running temps are OK, not sure if that was addressed -- engine not overheating, or running cold. Cold running could produce fouling.]
