91 200q power loss @ WOT

[RRFS32D@PRODIGY.COM (MR JERRY BEER)]

Need collective wisdom.  After various testing, car loses power 
(mildly) when foot to the floor.  Feels like a mild surge, or knock 
sensors reducing timing or boost.  If I lift up a little, car pulls 
much better.  Only last 2-3 psi of boost affected.  Before TAP spring 
this occured @1.6-1.7 bar.  Now it happens @1.8-1.9bar.  Seems to 
rule out ignition and fuel delivery.  Car was recently adjusted on 4 
gas flow meter for emissions.  Could it be WOT switch?  I thought WOT 
closed looped the OXS, thereby running rich. Feels like car going 
lean,  not rich.  OXS replaced 1 year ago and seems to work(notice a 
slight stumble about 30 seconds after start up which I assume is OXS 
kicking in).  Disconnecting OXS doesn't affect WOT problem.  Can't 
believe it is too rich @1.9 bar.  Car pulls great until WOT and heavy 
boost. Bad WOT switch?  Air mass sensor/potentiometer?  All 
suggestions welcomed.  TIA
Regards,
Jerry
91 200STEADIA3qw