88' 300E runs rough at low speed all of a sudden

[land sea air]

Hello all, hopefully you can help me solve this issue in short order as I'm trying to sell this car ASAP. Here's the story: car was running fine save for a dying battery. Alternator tested fine so I got a good strong used MB battery from Johnhef, installed with no issues, after a short fairly high speed highway run from the junkyard to his house later in the day the car started running poorly at lower speeds (especially idle "put put put put") still runs fairly smooth at highway speeds and such. I replaced the cap and rotor to no avail and the plugs and wires are both fairly new and in good condition. I suspect a possible 02 sensor problem but I'm wondering if this could cause this sort of problem. it was replaced about 2 years shortly after I bought the car and I thought it should last longer than that. I did unplug it while the car was running to see if it ran worse and I didn't detect any changes besides the CEL coming on (how do I clear this by the way).