G*d d**m p0420

[aapplebey]

I have a 96 318i

I am getting the very popular p420 code and its a real biyatch. I recently replaced my leaky exhaust manifold. hoping that it would be fine after that but it was not. 50 miles later the light and code resurfaced. I changed the o2 sensor at the front of the cat. hoped that would do the trick it actually ran for about 150 miles before the code popped up again. What can i do? i have noticed that it idles a little rough at times and this has me thinking about the mixture being rich. any ideas?