Fluctuating RPMs on twin 250 EFIs

[aquadj]

I have twin 1998 250 hp ox66 EFI motors. Operating below 2200 RPMs or over 3700 RPMs to 5100 RPMs both motors run perfectly. Running between 2200 - 3700 there is up to a 400 RPM fluctuation that sometimes occurs on the motors seperately. It sometimes happens on one or both motors, or not at all. I'm thinking O2 sensors...any suggestions?

[boats.net]

Cleaning the O2 sensors would probably solve problem, try that first.