[RWebb]

one tidbit to add -- as the O2 sensor ages, the response slows down -- possible that fluctuations are due to the computer trying to compensate for slow response

the sensors last a LONG time - but are simple to replace and not too spendy; also be sure the coax cable is in good shape (I'm not familiar with the new-fangled 911s but the above is based on your typical German car of this vintage)