Well, the stock Rhino should see around 42 mph for a top end speed. the speed limiter then kicks in and will remove spark to the plug. two things come to mind. The fuel filter is plugged (crap in the tank from poor QA)  and or the gas tank vent line being plugged or pinched. Both happened to me and caused backfire and shutdown problems. The tank vent only shows up after running for awhile. The other possible problem could be the manual choke is stuck.