Get a digital storage oscilloscope and hook it up to your O2 sensors....take a look at the cross counts on the scope and make sure it's switching from rich to lean evenly. It should cross the .450mv mark at least 2 times per second....also if the voltage dips into the negative value then you have a bad sensor. Make sure you test and check other sensors first because based on the order of influence the O2 sensor isn't the most important one. Order of air/fuel influence is : tach, maf, ect, iat, o2.