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.I removed the carb float bowel and it seems to be full of fuel and fuel is flowing well from the fuel tank. Thanks for any ideas