Grasshopper 616 - Slow and getting slower

My mower works ok on straight paths but as soon as I start up an incline, it begins to slow down. My backyard is not perfectly flat but it is not overly steep or rolling. I would describe it as "5-10%" slope but more flat than slope. And it appears to be faster in reverse than it is in forward, even on a straight line. I got this mower 4-6 weeks ago (don't know the history and don't have service records) also I cut around 2.5 acres - Need some suggestions.

Model #: 616 - 16 hp
Model Year: 1995 or 1996
Serial #: 340931

Any suggestions would be great,
Thanks, SLS

I really like those, and took a good look at a used one which did exactly the same thing. When I first started moving I could go up a little incline very quickly, but after a few trips around the fellows yard the speed uphill dropped alot. The owner said it needed a hydraulic service, I did not buy it and don't know if that could have been the issue.

I remember that the mower gear box was shaft drive but I don't recall if the pumps/motors were-could a belt be slipping if the transfer is by belt?-fluids at operating levels?

Its a fantastic unit, my money is that it is a simple fix but keep in mind that the machine is a beast, isn't yours over 1/2 ton dead weight, add yourself and then the work being done with the deck engaged and perhaps with 16? hp this is 'normal'..

Robert, thanks for the suggestion. I got an email from Grasshopper tech support and they suggest that the transmission belt may be slipping (could be worn, stretched) so I'm calling my local grasshopper suppliers. Thanks for your input

From Grasshopper - "This sounds like a possible slipping transmission belt. A slipping belt will result in weakness on both sides. If the belt is slipping, also check the conditions of the sheaves the belt rides on."

Sounds like it needs a new belt.