I have a 61" Ferris with the Kawaski engine. Also have a lot of very rough ground and a lot of side slopes. I would also strongly recommend purchasing the optional seat suspension. It made a world of difference.  I think the problem is that the mower works so well at faster speeds I tend to run it much faster than I have other mowers. On rough ground at higher forward speeds you still need a bit more isolation in the seat. Your back is worth the difference in cost.