Z518 with the non Pro Xs 4 stroke 150. Top speed used to be 55-56 now struggles to get to 50-51. I am looking for any ideas or help that I can get on this it would be greatly appreciated. I had to have lower unit replaced and some engine work done a few months ago. Afterwards could only run 46-49mph and would drop a couple hundred rpm’s. Took to shop and everything checked out ok and they changed fuel filter which seemed to fix the rpm’s issue and it stays at 5800 at WOT now. They lake tested and hit 55mph no problem. I picked up boat and could only hit 50-51. Finally after couple days of running it, while going down lake RPM’s 5800 it picked up and shot to 55.6 in and instant like you shifted to another gear. Rpm’s still stayed at 5800. Had to throttle down and could never get it to do it again. Back to 50-51 tops. Shop is a good shop so please no dogging them I know this is intermittent problem and going to be hard to find, I am just hoping somebody has had same issue and could steer me in right direction. Thanks.