Problems with Snapper rider model 2809-15BE

I got a Snapper rear engine rider with 10 hp B& S model 28A707-0122-02. I chged oil,new spark plug,new tires,new air filter. the mower runs good on level ground but when I go across a hillside (not straight up and down a hill). It cuts off ,pops and stops running. I can usually start it right back but will only run a few seconds unless you return it to level ground. Thanks for any ideas