If there is only 1/8" pitch in your deck it probably drops to "level" (no pitch) when you get on the mower since the weight of the operator adds more weight to the rear tires than the front. I'd try resetting the deck so that it has 1/4" deck pitch with you on the mower.
Good luck! Stan
Shut 'er down Buford....she's slangin' oil !!!!