no mid range rpm

[bobtootles]

I have an old but new 150 hpdi that I put on an 18.6 whaler. Only 10 hours on it. Engine idles fine has a top end of 5700 rpm with a 17 pitch prop but i can not hold any rpm between 2000 and 3000. If it is rough I either have to idle or plane at 3000 rpm. could this be because it is under proped and the computer won't allow the boat to plow at these lower rpm's

[boats.net]

Probably wont be able to hold rpm's when plowing, as the boat starts to plain(decreasing load on engine) rpm's will increase.