I like Paul's idea about the air pump. If it was engaging while warm, it was also probably activating the check valve. If you have since disconnected the pump clutch you still may have the check valve opening. Paul is right that the exhaust will aspirate air in through the pump if that valve goes open. This will fool the O2 sensor into thinking you have a lean mixture and the thing will go full rich trying to compensate.
Bosch Master, ASE Master, L1
33 years MB technician