I may know what is going on here. Now keep in mind I have a stillen supercharger but I had some of the same codes as you. and I would try this before trying to change the upstream o2 sensor since it has probably welded itself to the header and there is no way of removing it without destroying the header etc. the problem is the placement of the intake filters and how close the MAF are to the filter. there is a little plate that comes with the kit that is supposed to block air/water or whatever from directly hitting the filters, if you are missing the plate like I was I would get the AF jumping all over the place and it would create a limp mode issue. Here is how you can tell if this is what is happening when tuning and you have the fan on in the front of the car for cooling turn the fan off and see if the air fuel settles down. then turn it back on it will jump all over the place. then put a piece of cardboard in front of the filters to block direct air and try it again and you will see the difference. My fix was to turn the intakes around to fit under the drivers headlight in that dead space so they could get air when needed but be shielded from direct air intake.
what happens is especially when you are using a fan to cool during a tuning session it creates a ram air effect. when the car is not under load (ie giving it gas) the engine does not require a lot of air flow but the cooling fan basically force feeds the engine air causing the crazy numbers. Take the fan away you get a more accurate reading when not under load. So try shielding the air away from the filters and you should be ok. I just decided to take it to the next level and move the filters off to the side to prevent this effect. it allows the filters to get the air they need without blocking the air flow going throught the front of the car. I am not sure if Stillen realizes this problem but in some way I think they did and that is why there is that deflector on there but when you go to do a custome tune and are using a high output fan to cool it creates havoc and causes these problems. Mine consisted of the car when crusing at high speed the air is forced into the engine causing a limp mode of sorts and when I would fall under 2500rpm it would not allow me to go above that until I turn the car off for 30 seconds to reset the computer. It also caused multiple misfire codes and bank to rich codes to pop on mine since finding this out I have not had any of these problems since.
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