New PRM 90 Slipping?

[Obi Wan]

I have finally installed my new PRM 90 gearbox to replace the Hurth that had the clutch fail) with my newly overhauled engine. The gear box is installed correctly with the shift lever throws adjusted correctly. It has ATF filled to the correct level. Yet, it seems to be slipping. It almost couldn't back the boat up into a breeze and I almost ran into the front of my slip when I used reverse to stop on the sea trial because the boat didn't respond as it did before. It also doesn't accelerate forward like it used to (with my old transmission before it failed). Does anyone know if there is a break-in time when it slips or does this sound like a problem? Hopefully others who have installed a PRM 90 can chime in with their experience.