Paul, So you can actually feel this when it happens or see a loss of torque on the dyno? Give me an idea of how much time in the log is passing by when it happens. What has been done in the cal. Have you tried a new TIS flash on it and then start over to see if it repeats?
Don't forget that if the WB is some distance from the heads, there will be a delay between the commanded AFR and the measured exhaust AFR.
I don't think it would be this, he is creeping into PE and it still does it. If he is feeling it when it goes lean and the WB is seeing it, that is a fairly long time so to speak. There could be something going on in a table you are not seeing.