Audi Repair Questions? Ask a Mechanic for Answers ASAP
I would check the intake hoses, to make sure they are all tight. It sounds like you are developing a leak under pressure on that bank. Either that, or your timing is starting to stray off on that bank, maybe check to be sure the tensioner is OK.
The rear O2 sensors monitor the operation of your catalytic converter. More than likely the sensor is showing a fault due to the rich in burned mixture it keeps seeing, and may also be getting covered in a layer of sooty residue from partially or unburned fuel.