How about another uninformed opinion?
Check vacuum lines and inspect for any breaches in and around the intakes.
If the combustion chamber is sucking more air than what would normally be "metered", that would cause a lean condition, especially under "load".
I would presume that the engine management system would compensate for that, but it would be something I would inspect.
BTW, how long has this been occurring? Any recent repairs done in the engine bay prior to the incident?
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car