In my car every once in a while it will go pig rich (pegged at 10.0 AFR like yours) during a WOT pull. Won't do it before or after, and if I let off and go back WOT it often goes back to 'normal'...never did figure it out, as it was so sporadic and infrequent that I didn't have much data to go with and it was not high on my list of things to deal with.

But, I am curious to see what is causing yours and if it could be causing my car to do the same.