O2 Sensor- 95 E-36 318ti

[JHahn22@xxxxxxx]

Hello Folks,
My 95 318ti (M42 motor) automatic (135k mi) has been giving me a check engine 
light anytime I let it idle for more than a minute or so.  It also runs a tad 
rough when it gives me the light.  Hooking it up to the computer told me that 
the oxygen sensor was bad, which was what I expected, so I replaced the 
sensor.  That may have helped out for a little while (a day or so at most), 
but the situation is now the same.  I haven't checked the code for it again, 
but it gives all the same symptoms, so I'm expecting the same code.  
Incidentally, after the light has come on, even the slightest tap on the gas 
pedal makes it go away for about ten secs. or so, and then the roughness 
gradually returns and the light comes back on.  It feels like it's idling too 
low, and raising the RPMs even by 100 or less for a split second is enough to 
make it happy for a bit.  Usually, at stop lights, where I've had the RPMs up 
for a while before idling, it will take a minute or so before the symptoms 
occur. 
Question is, what do I go after now?  Cat (hope not)?  Other tune up stuff?  
I don't know how old the plugs and air filter are, so this should be 
addressed anyway, but what else should I look for.  I don't ever put anything 
less than 91 octane in(the middle-priced type), and I haven't noticed any 
difference between 91 and 93 oct.
Thanks in advance,
Jake Hahn
95 318ti
College Station, TX

------------------------------