Genshie,
Not sure if this will help but, the other day my truck backfired on startup, and would only do the same thing yours is doing(start up,die after a few seconds,start up again), I looked around and found out that it had blown the hose from the intake to the breather box (not sure of the right name) off of the intake side. I put it back on and tightened the clamp and it fired right up and never gave me another problem...hope this helps, Dave