Steve, When the car is seeking after warm up, it's usually the air flow meter needs to be adjusted. If the 02 sensor wasn't working, then is normally wouldn't seek. What it's trying to do is find the right air/fuel mixture. Use a 3mm long handle allen wrench and adjust only when the engine is at operating temp. Wrench cost about $2.50 at Sears, the rest is time. If you replace the $43.00,, 02 sensor, Then the adjustment may have to be done again. John hervey http://www.specialtauto.com/ << Now my questions: 1. Could there be something wrong in my Lambda circuit causing this such as a bad O2 sensor? No, It sounds like it's working. 2. Could my frequency valve be bad? No, It sounds like it's working also from your description. >>