Sounds like your torso and the seat are not a good match. I'm assuming you have made whatever adjustments that are available to the seat to get the best fit. If that's not working, then you may need something different. It could also be your own physiology. I sit at a desk for a living and had to get a chair different from everyone else in the company because the "normal" chair the company uses was killing me after a few hours of sitting regardless of how well I adjusted it. I ended up getting a Herman Miller Aero chair which fells like it's not there and I'm floating. I'm thinking you may have a similar issue.