Strange Intermittent idle Problem

First _ I realize that idle problems can be as common as oil leaks on these trucks : ) I've ruled out the common stuff- HTL's, cleaned the coil pack connections New Plugs, air filter, fuel filter etc- The truck starts and runs great (for a 210k 98) after a a few cycles to school and the store and some stop and go traffic it starts to run rough not a shaking misfire but if you hold your hand under the exhaust it is miss firing- like a cough- no check engine light - my original thought was a bad tank of gas or injector- but I've run through several now- ran some injector cleaner through also- Now I'm thinking maybe a vacuum or evaporative problem? I read somewhere that a purge / vacuum valve is in line with the fuel system? Also perhaps a bad O2 sensor- they heat up over time? The thing is that most of the time she runs smooth as silk then the coughs- I thought also the idle control valve might need cleaning but that seems unlikely as the problem is intermittent and idles rock solid when not coughing- Any thoughts or directions of pursuit appreciated

Peter