Help! - no idle when warm!

[Mark Graham]

Have an 86 5000 QT, and recent warm temperatures up above 85 F. Car runs fine when cold. In afternoon, going home, and car up to temp, will stop for a stop light, and the engine will die. It will restart immediately with the key.  If I keep the rev's up above 700 - 800 rpms, the motor keeps running. Do I need to adjust something (WOT?) Could it be this new 10% ethanol blended gas? Any pointers would be greatly appreciated.  Haven't found any bad or leaking vaccum hoses?
Thanks in advance!