I recently had a similar problem, and found a solution (well, at least it worked for me). It turns out that the gas cap vent was the issue, or the lack of venting to be more to the point. Somehow the vent in the cap stopped working. When I removed the cap, there was a huge inrush of air. Ah HA! So I put the cap back on loosely, so as to keep dirt/etc out and the gas in, but to permit "venting". Mowed the entire lawn (hilly, 4 acres, dips, trees, etc) without incident. Problem solved, now have to order a $10 gas cap as they don't stock it locally.
Oh, and I'm running a 2007 DGS6500 26hp Kohler 54" deck.