Why does my jeep always die while idling?

I have a 1992 jeep cherokee laredo. straight 6. 4.0 liter. whenever i'm at a stoplight or drive thru idling my jeep's rpm get really low and it eventually dies. what could be causing this? my dad and i already adjusted the idle speed but it still does it anyways. what do u think is causing the problem? what should i do to fix it? how much do u think it will cost?