outboard motor power problem

I have an 88 mercury 50 I just had rebuilt. It starts well, idles well, pumps water etc. The only thing it doesn't do is push the boat if the rpms go over 2500. At lower rpm the boat moves forward slowly, at higher throttle, the engive revs up but there is no power to move the boat. I've looked at the prop, it looks good - no wear on the splines - it's only year old. I need some other ideas before I have to have them take it apart again!!