no exhaust leaks? sound stupid i know, but i have seen on vw's, having a exhaust leak near the o2 (have seen it a few inches behind the o2 even do this) it creates a vacuum, sucks fresh air INTO the exhaust, and the 02 will read that fresh air and call it a lean condition....probably not your problem but throwing it out there....
how did the plugs look? if its o2 or exhaust related, they will be black or look/smell like they were running rich. the computer will adjust for lean/rich to a point then throw a code. so if they look good or light, you know you are lean and its the intake side, if they are dark then the computer is reading the o2, and exhaust leak and adding fuel when it doesn't need it, so you will get unburned fuel and that can cause a misfire.....the plugs are an awesome way to figure out whats going on, even on an electronic controlled engine....
Plugs look good and light. I dont think there are any major leaks. It is an old exhaust but seems like it is solid or patched where it needed it. Nothing really obvious that i can hear or feel.