2000 sable LS miss while idling in drive

2000 sable ls w/duratec dohc 24v. missing intermittently while idling in drive. kinda seems to coincide with heat/ac running but not 100% sure if that's the only time it happens or even the cause of it happening. garage recently replaced upper and lower intake manifold gaskets as well as IACV, and EGR. (note: the idle anomaly preceded the above mentioned repairs and still continues)
i inherited this vehicle from a relative and have already dumped a lot of ca$h into it. would like to hear anyone's thoughts on cause and possible remedy's to this annoyance.
5/2/2012
Still looking for answers. any ideas?