Intermittent Problem - need ideas!

[BuilderFL]

Motor was working poorly, assumed that I had a power pack or coil problem (which I***8217;m getting thru bigshrimpin), however I cleaned the carburetors and she started up right away, with higher rpm that it***8217;s had been set at. I figured, the cleaning did her good. For good measures, a replaced the fuel pump diaphragm kit, the fuel filter/water separator, adjusted the rpm to 900 and the motor was working perfect. Took her out yesterday, nice idle rpm at the ramp and she ran like never before, doing about 42 mph, which for a heavy boat like mine, it***8217;s very good. Ran for about half an hour and when I reached a no wake zone that I slowed down, the motor turned off. She would start, but it would not go beyond 1,200 rpm. Every time I tried, she would bog down. I did notice that if I pressed the choke while running, she would increase rpm rather than decrease. Pushing the choke off and on in 15 seconds intervals, she would gradually increase the rpm to running speed, however the rpm would intermittently drop and go back up by about 300 rpm. I did make it back to the marina and she would not idle, increased the rpm and noticed that she would start, but the moment I put her in gear, she would shut down. She started again back at home, but sounds like it***8217;s running at reduced power.
Any ideas?
1989 Mercury 200HP
Thanks,