A hydro on a Cub isn't like a cruse controll, it is more like a hydraulic cillinder.
If you try to pick up more weight with a hydraulic cillinder you need to push the lever a little further. Same with a Hydro Cub, pulling more weight or going uphill requires more power.
If you are on flat ground and your speed changes by itself, then you need to look here
http://www.onlycubcadets.net/forum/showthread.php?t=43
http://cubfaq.com/hydrolurch.html
http://cubfaq.com/trunionrepair.html