O2S or EGR

Have recently purchesed a 1990 astro with 200k miles. Only have a few problems, trying to knock them out one at a time. Have taken care of the air cond upgrade, now I'm dealing with a rough idle only at warm speeds. I have read many of the posts here, have seen the posts for the EGR valve. My vin is a Z. This may still be a problem. Have checked the plugs, wires. Replaced rotor button and distributor cap, both were very bad. Runs better and starts faster now, but still when warm idle is rough. Also replaced fuel filter. I know the O2S sensor will cause rough idle and is only monitored when warm. Any ideas as to determine which is the prob, or if it might be something else.

Try replacing the coolant temp sensor (not coolant temp sender). It should be only $8 or so.