Your problem is the vent in the fuel cap is clogged. Try running the bike on the stand for a while and see if it just dies, then run it with the fuel cap off. If it keeps running then the problem is a clogged vent. Take the cap and carefully look for the small hole and ream it out with the end of a paper clip.
Had the problem with a new Tomos that just kept dying... I figured it out when the service tech spent 3 hours screwing around with it.