8.1's high rev power fluctution?

[AlGallinaro]

Running 8.1's on a 2002 340 SD 700 hours. Don't bring it up 3800 -4200 rpms very often and did and noticed the starboard engine tack bouncing and felt an intermittent power back off. Its not noticed at 3000 0r 3200rpms or at any speed. Changed the fuel fliter, have been treating the gas, generators on that tank and it runs well. Changed the plugs in the spring. Anyone have a most likely suspect? i.e. fuel pump, ignition, injectors. Don't want to randomly replace parts, might have to call a marine mechanic in seems like it could be heading to a bigger failure...Thanks