First suspect is the vent line that others have mentioned. By the way, the dingus at the end of the hose on non-California bikes is a rollover valve to keep the tank from draining through the vent if the bike falls over.
There is a very small chance that you have an ignition controller with a heat problem. You can check for this with one of the cheap, otherwise useless, neon timing lights from the bargain table of the local autoparts store. These things are a very electronics friendly way of checking for a spark.
