Itzroyal, I figured out yer problem. It's the high-flow cat throwing off the reading on your secondary O2 sensor. You can fix it for like $4. Here's a write-up with no pictures. You need 2 spark plug non foulers, and drill one of them out. Connect them together, then connect them to the secondary O2 sensor. Bam! No more issues. It spaces your O2 sensor away from the cat a little, and fixes the P0420, which is probably causing the other random misfire codes.
Mechanical o2 Fix - evolutionm.net
