I'm sitting here imagining you stropping from right to left, and I feel this could be part of the problem. Not that you can't keep the razor flat on the leather while making strokes this way, but I don't think this method is one that I would teach. I believe you would better serve your strokes with more control and concistancy by standing with the strop more in line with your body(you would be stroking toward you and away from you). This would, also, help keep your blade more level. I would get a hook from Wal-Mart, or whomever, and mount it either higher or lower than it is now. This way your stroke would be more up and down and close to 90 degrees rather that doing a horizonal 180 degrees.
IMO, I really believe this would help.
(I had difficulty explaining what I meant) I hope you understand what I am saying -
Steve