I helped with a 7M setup for Bigaaron a while back that you would have sworn was an ignition problem. We scoped it...checked out every possible ignition issue...you name it. It would stumble and stutter very badly when it came into boost. There was no indication that there was anything amiss as far as the lambda or anything...no erratic changes or anything.
In the end, the fuel mapping was just wrong. It had been "tuned" (using that term very loosely) previously by a very well-known import racer/tuner. The map looked like garbage (very chopped-up with peaks and valleys everywhere), but I assumed that he knew what he was doing. In the end, it just needed to be tuned properly. That could be the case with yours as well.