what engine do you have? I'd make sure the car is fully tuned up (plugs wires cap rotor fuel and air filters). past that it could be the distributor on its way out, timing, egr clogged iac or tps problems too
AFAIK, there's one per header for OBD-I cars plus an extra post-cat one for OBD-IIwhich o2 sensors ... sorry for mynoobness but isnt there a whole bunch?