1984 Johnson 70, Sometimes runs good, sometimes not?

I have a 1984 Johnson 70, 3cy 2 stroke.
I have had the same problem for about a month now.
Sometimes it runs great, sometimes not.
Last weekend we took it out all day Saturday, started right up, idles nice, good power. We ended up running about 25 miles total.

The next day we took it out, it did not want to start, once it did start it ran ok for just a short bit, then it had no power over 1/2 throttle, and if you gave it any more it would just die. (We were running on the same tank of fuel as the day before, no change). Once it did get up to speed it would act like I was giving it full fuel then cutting it back to ½ with no change in throttle by me.

This same problem has been going on for over a month, runs great one day then crap the next.
Plugs are new, fuel tanks are clean, new fuel filters, screen in motor pump is clean, ran some seafoam through it with no change, took bowels off of all 3 cabs and all are clean. I have no idea what the problem could be, local guy is booked for the next 2 weeks and he is the only one I trust around here.
Any ideas?

An intermittent problem such as you describe will be difficult to diagnose unless it's acting up when being worked on, however................

Inspect the stator under the flywheel closely to see if it's cracked anywhere, and if any of the three large coils within it might be leaking a sticky looking substance down on the timer base and powerhead. It is is in this condition, that would result in a AC voltage drop to the powerpack's capacitor.

Also, check for a intermittent short in the ignition switch that could be affecting the powerpack...... remove the black/yellow wire (kill circuit) from either the ignition switch of the powerpack (whichever is easier). If this cures the problem, replace the igniiton switch.

NOTE: With the black/yellow wire removed, you will NOT be able to shut the engine down via the ignition key.