What causes a 2002 Toyota Sienna to start but not stay running?

twice Toyota replaced the idling valve assembly in my 2001 sienna van, under the 36,000 mi warranty, since then I've had it replaced another 2 times. the mechanics say it's a defect in camryand sienna vans. so my last mechanic advised me to use better brand gas like chevron, which i guess helps coz i used to put any type from arco, vons, Costco, generic types. when the idling assy is replaced, it starts and idles fine, assy alone costs about $220-260+