Note that when this happens, the O2 will foul in as little as 10 minutes. When you install the new or cleaned O2, does the car run ok for a little while, then go back to the "status quo"?

Although not on this motor I went through a nearly indentical problem on my 16V(also "KE")..turned out it was the ecu. However in the process, I messed up three O2 sensors....I was able to recover the last one by cleaning it on the bench and re-using it....what I did try was to disconnect the eha, then re-adjust the mixture, and ran the car tuned "manually". Although not perfect it allowed me to run semi normally over 2000 rpms.

Bottom line...we tried another ecu from a different car..and it solved the problem(after we retuned the air fuel mix..leaning it way back down to the correct readings with everything reconnected)...Actually we tried an ecu from a 2.6 W201, and guess what..the car runs fine with that too...strange.