Just had something similar happen to me, turned out that a piece of something had gotten under the main jet. Looked like some rubber sealant or Teflon. It would idle fine, then when I was about a mile down the road It would quit. As soon as it quit I think the obstruction would drop back down out of the way. Then it would start right up. I was already having problems getting the carb to quit leaking, so I stumbled on the cure accidentally.
