Something is not correct for sure. You should be running about 60 or so with 23 pitch at 5 grand. What is the condition of your props? Check each blade very carefully for any cracks, flolded over blade, dings..... What kind of prop? If has a changeable hub make sure that is not messed up as well. I doubt it is your coupler as from what I have heard when they go they go. How much are you triming the boat? Are you trimming the same amount? On the motors are they carb or EFI? Have you checked your fuel filters and so on?