In my opinion until you figure out what is causing the richness on the one side you will probably continue to dirty up plugs. If it were me and I didnt replace them I would put the good looking ones on the rich side and clean the rich ones up a bit and run them on the other side. Typically I am the guy who would change them and have the others for an on hand set or change four of them. I am also very leary of using any plug I drop and cautious of cracking one on tightening. Haha ive had a couple plug issues so im cautious. You know best if you can afford another set or not and how bad it was to remove-install plugs. For certain be sure to check for any exhaust leaks on that side and address them before you gunk your plugs up. Take a look at you o2 sensors and connections on that side as well and also looking for fuel in the line goin to your fpr as fred and bw hunter advised. Those were two things that will effect the whole bank. Also if you can scan it I would and if I were you and plan to keep car I would consider getting the cable and software to scan log and flash as the obd1 isnt near the investment mine was. Scanning it might show what the o2 sensors are doing and could determine thats your problem or your having a leak in front of them causing false readings(dont think scan would show that). I dont know much about all that but some of the guys on here know their stuff so dont ignore the scan. Do as some of the experienced people advise as alot of people just start throwing money at things and dont listen. If you have checked for exhaust leaks tell us and a quick statement of how you did so and same with anything you try. That will give they people a chance to offer up a better or more comprehensive way to do it. In my opinion there probably isnt that many things that would effect one side like your issue so do what has been advised and report back and once the common or probable things are checked then they may think a little deeper and offer more direction. Good luck. Once you get her goin good I would consider a good tune flashed into you pcm either dynotune or a tune with some logging for fine tuning. I would assume you would pick up a little performance and perhaps fuel mileage too.

 

Did you switch the O2 sensor like advised? Did you use a sealant on your headers with your gasket at the head? Yes you can change the plugs or just clean them up.