[Biturbos4] B5 V6 "unilateral" misfiring

[Chris Newbold]

The patient: 2001 S4 with the 2.7 biturbo V6; I was deliberately vague
in the subject about the car in question because I don't think it's a
unique S4 issue...

The problem: occasional rough idle; stored misfire codes as read by
VAG-COM. Using VAG-COM to read the cylinder bank-specific misfire
codes with the engine idling reveals that cylinders 4, 5, 6 (bank 2,
US driver's side) are racking up large numbers of misfires--in excess
of the factory specification (Bentley says values should be < 15 per
cylinder). Cylinders 1,2, 3 (bank 1) are virtually misfire-free.

Stuff I've tried:

1) All new spark plugs; installed about 3000 miles back; much smoother
over-all power delivery, but still misfiring.

2) Rearranging coil packs; seemed unlikely for all three on one side
to go bad at once, but I swapped one or two from side-to-side;
misfires remained in bank 2 with bank 1 remaining clean.

3) Swapping ignition output-stage amplifiers; these are the
semiconductor widgets mounted to a heat sink on the airbox; there's
one for each cylinder bank. Swapping did not change the observed
behavior: bank 2 continued to misfire.

4) Ran though Bentley diagnostics for the MAF and oxygen sensors;
everything checked out within factory specifications. The only strange
observation is that the mixture for bank 2 where the misfires are is
being leaned-out by the O2 sensors right to the limit of the factory
specification--both the "additive" value (for idle) and the
"multiplicative" value (for full-RPM range operation) are around
-3.5%. It isn't clear to me if this is symptom (misfiring => unburned
fuel => need to lean out) or if this is part of the cause.

I'm not sure what to try next; clearly something is not right here,
but there isn't any smoking gun. It's really odd that it's just one
side of the motor... Any ideas would be much appreciated. Thanks.

-Chris
2001.5 S4 Avant 6spd
2004 allroad 2.7T