Are you sure the prop is clean - and properly sized/pitched? Are you starting to develop black smoke at that rpm?? Is this a new phenomenon?
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)