Another possibility is wires, I ran into a similar issue years back same RPM ranges. The engine had just had new plugs and points having just come out of storage. I ran the boat on another tank suspecting poor or contaminated fuel, rebuilt the carb, did vacuum checks you name it. Decided to try a set of wires problem solved, I surmised that in that RPM range getting onto plane was the heaviest load and the ignition just would not carry it.
No matter what your beliefs are "GOD BLESS AMERICA"
Fully retired marine tech near 60 years in the biz.