Motor stall beyond 3000 rpm-cuts out

[BuilderFL]

1989 Mercury Black Max 200 HP
I would take the boat out and I could get on plane with no problems, but after a while, the motor would bog out, and loose power, it would run about 9 mph and turn 2500 rpm. There will be times that after 5-10 minutes later, the motor will kick in and start running great. Then it would loose power again and if I push beyond 3000 rpm, it would stall and just cuts out, however it's fine if I run at lower rpm.
Any suggestions as to the problem?
Thanks!