I have a 99, EX, auto, V6 with 113k. For the last year, I have been pulling my hair out trying to fix a bad idle issue. I have adjusted the valves, repalced the cap, rotor, wires, cap, cleaned the IAC, checked the EGR and reamed out the upper plenum, bled the cooling system. Still after all this it stalled at idle when hot and threw out msifire codes for all cylinders.

I gave up but last night, I replaced the failing ignition switch, common problem, and the 02 sensor and it went away!!!!

I am thinking it was the 02 sensor because it idled OK when cold and only died when hot. That got me thinking that maybe it was a closed vs open loop issue. When hot, the fuel delivery is controlled by the O2s, if they are degraded, but not totally defective they may not deliver enough fuel to sustain the motor when the throttle is closed. An idle is analogous to a pilot light on a furnace it just keeps the motor running so you dont have to restart everytime you stop. These motors are very precisely programmed to run in a tight air/fuel range so even a small error in delivery at idle will cause a miss or a stall. At least that's my take.

I dont think the ignition switch had anything to do with it but you never know failing ignitions do crazy things. I had a Pontiac Grand Prix once with a failing ignition switch and it would make all the gauges on the dash go crazy? Maybe someone with a little electrical background could chime in here. None the less it's idling perfect