Well, this is a very hot topic isn't it.........and haven't gotten any help here so far but I'm not giving up

I've replaced the O2 sensor (went with the universal one) and I'm still getting codes for the O2 sensor (code 1)and the EGR Valve (code 12). A friend said there were a lot of other things that could cause it to code for the sensor (bad plug for one) and also said the cat could be clogged. The car supposedly has a "high flow cat", DC header (both between 2 and 3 yrs old and probably 25k miles). They symptoms are intermittent rough idle, but mostly the car misses when under a light load when accelerating. If I go ahead and mash down on it, it smooths out (usually after about 3200 RPM). When it's cold it seems to run fine most of the time and the problems start after the engine warms up to normal operating temp.

Any thoughts anyone?