It's in the tune. I had the same problem. I'm assuming you got a tune since you did all the other; if you didn't then you need to. Again I'm going to assume you have a tune since you say it is running in open loop. If you are then why? In open loop the O2 sensors are not read at all. You should run it in closed loop with rear O2 sensors off. The following is an explanation of what is most likely going on if you are actually running in a closed loop. The reason you get it intermittently is because the ECM runs a self test ever so often on the sensors on start up. The O2 sensor have to be heated to a certain range or the ECM will think they are faulty and throw the code. The reason they don't heat up fast enough is because they are further down the manifolds in the collector of the longtubes. The time delay for heat up can be extended in the tune. It took me 4 email tunes (American Muscle sucks IMO and this was before John Lund started doing their tunes), 2 sets of O2 sensors, a couple months of headaches, and finally a custom dyno tune solve this. Most anything you search on this will say, bad sensor, vacuum leak, exhaust leak, bad MAF sensor, and on and on ... Bullshit! Check the simplest thing; exhaust leaks, if that ain't it (and it's probably not) it's in the tune. The reason it stumbles on itself is because it is actually running really rich. To prove this to me my tuner put the car on the dyno with all his equipment hooked up. Upon start up and idle we watched the A/F ratio peg his meter at 16.0 then the ECM compensates and dumps fuel into it. As we sat there watching the A/F ratio then dropped and pegged his meter at 10.0 the car almost died right there and the fumes from the fuel about ran us out. So get it tuned/re-tuned. If it is in open loop and you have this you have a bigger problem; as I said the O2 sensors are not read in open loop. Hope this helps.
BJ
