go and run

Good morning : I m new to Lawn World,and I need your help. I have a 2014 Craftsman G5100 with a little over 200 hours and it was sluggish in drive so I called the gent that helps me with some repairs and he told me that the newer Craftsman mowers did not have a belt adjustment so I put on a new belt, seemed ok at first but when you get on and incline it stops.I was wondering if any of you guys have experienced a situation like this? How do you tell if it is the belt slipping or the tranny is slipping .It has a hydrostatic transmission.

Is that the one with the 3 cylinder Kubota or the V twin Wisconsin ?
72" or 84" deck ?