I would agree with others to try shifting and throttle at the engine. As JimsCal said above, slipping would cause high revs. So, it is NOT slipping most of the time, it is grabbing.
But, if it is not a linkage adjustment:
I just recently had a similar problem on our boat
, and it turned out to be prop fowling, (turns out there has been a lot of that this year in Puget Sound). I could not get higher RPM, she would barely move, could not maneuver, engine blew black smoke. Diver scraped blades, (it had been out of the water
only 8 months prior), and VOILA!