1992 Toyota Camry O2 sensor/engine performance HEADSCRATCHE

Engine Performance problem
1992 Toyota Camry 6 cyl Front Wheel Drive Automatic 185K miles

Recently developed Rough, hard idle, white smoke with heavy unburned fuel odor, triple misfires at idle.
Check engine light comes on, I cleared codes by pulling neg. Battery cable to "reset" computer. Of course, check engine light eventually comes back.
1. I pulled the OBD I codes using the "paperclip jumper" method on the inside diagnostic connector, spit out Code 28 - O2 Sensor. Same code over and over after resetting C.E. Light.
2. I replaced both right and left bank O2 sensors (single wire, factory plug), cleared codes again, and after 4 or 5 on/off cycles and driving the engine seemed to have "re-learned" the proper mix and was running about 95% better, but not quite dialed in.
3. HOLEY MOLEY, SAME problem re-surfaces after about one week, just no check engine light yet. Rough out-of-sync idle in morning, feels "off" when driving and has slight surge when at steady speed.
4. Called "Yota" dealer, and asked Do I have a THIRD O2 sensor, 'cuz I ain't finding one! Dealer sez, Yes, based on your VIN #, and it's in the area of your Resonator pipe.
5. Went hand over hand from before CAT to muffler tailpipe, and I'm NOT finding a senor or tell-tale wire leading to sensor. Hmmmmm. I do believe a do NOT have 3rd sensor.

I realize there are a gazillion things that could CAUSE these symptoms, but WHY would O2 sensor replacement "clear up" problem temporarily, then re-surface?
My last post was over Airbag light, went 3 pages with 29 posts, and I believe we all learned something. Soooo, let's do it again! I await input/opinions - Thanks!