do the O2 diagnostic as per the helms or chiltons, to make sure your O2 heater is operational. the engine runs full rich when cold and accounts for it, but once warmed up it samples the O2 sensor...problem is that when your not making hot exhaust (idling and low speed driving), the O2 sensor isnt hot enough to give a stable output. this is where the heater circuit becomes necessary to prevent the car from running rich.

on the other hand when your on the highway and making horsepower the O2 is above 600*F from the EGT. so it reads accurately...

here: try taking the car out for a spirited drive...ie. get it over 4000 rpms on the highway for a few minutes, beat on it.

pull over
then see if it idles a little better.

if the car idles/runs better after being on the highway...
then it sounds like it could be the O2 heater or the heater circuit.

~kris