This happened to mine and it was a float issue in the carb, it stuck open some and on some hard roads, or bumps it would stall out and I would have to sit for a bit then it would start. Id check the fuel window in front see if u can see the fuel level at idle. if you cant see it stare at the window, and with a needlenose pinch the fuel line going into the carb. You might see the fuel level come down, if so your float is stuck open. other things to check is choke linkage, vac lines.....