The throttle body gets gummed up, so enough air doesn't bypass the
throttle plate at idle and the car stalls. Try removing and cleaning
the throttle body. While you're doing that, clean out the air passage
in the idle speed motor.
There are good instructions at:
http://www.brickboard.com/FAQ/700-900/EnginePerformanceSymptoms.htm#PoorPerformanceIdleSurgeandHuntDirtyThrottleBody
