Also check the gas cap vent to be sure it isn't plugged. My B died suddenly after running for a while and wouldn't start. Turned out to be the vent in the cap. Also look down into the tank with a flashlight. There might be something floating around in there that blocks the outlet periodically and causes your problem. I've heard stories about rags, bugs (big ones) and just rust/dirt causing this same problem. Duey's suggestion to check the anti-surge spring is a good one too.
Good luck, and let us know what you find,