I have a 2000 MerCruiser Horizon V-8. It refuses to achieve greater than
2800 RPM (3200 RPM if the Fire arrestor is removed). Top end has been
5,000 RPM. In fact, in neutral, it will rev at 5,000 RPM where it seems
to be limited by a governor.

This problem started to occur several weeks ago and would clear up when I
ran at 3/4 throttle for awhile, making a step change from 2800 RPM to 3000
RPM at 3/4 throttle and achieving 4500 to 5000 RPM at full throttle. As
time went on, the problem got worse, taking longer to make the step change
from 2800 to 3000 RPM and now I can only get 2800 RPM at full throttle.

Any (helpful) suggestions would be appreciated.

Thanks,
Tom