clogged pilot. do you have an inline filter for fuel?
I had a similar problem where the bike would run, then randomly die after 20 min or so and wouldn't start back up unless i held the trottle open. i cleaned the carb and it still did it, took it to a shop and they cleaned it with a machine and got a piece of "trash" out of it. they also put in an inline filter and its worked great since then.