look for some clues,

what's the o2 voltage when it falls on its face? steady high or low?

a severe ignition miss or a fuel starvation issue usually throws your narrowband o2 lean at WOT (normally narrowband should pretty much always be rich at full throttle)

you've been playing with your TPS, does it get a decent voltage at full throttle?

what's your MAP at heavy throttle like that? should be pegged at about barometric pressure, i'd hope, but is it solid or does it fluctuate?

if you post a datalog maybe someone will have some other ideas