1993 750iL (prod. 12/1992) increasingly went into limp mode over about a 15 minute span, starting out intermittent and ending up full time limp (low power, runs rough, etc.)

0) Reading the codes with the Peake shows only O2 sensor (for both banks).

1) Disconnecting the right MAF had no effect, meaning it still starts and runs rough. Disconnecting the left MAF and she won't start.

2) Putting a timing light on the right coil wire shows intermittent pulses at idle and no pulses at all above about 1K rpm. Checking the left coil wire shows strong consistent pulses.

3) Both distributor caps, rotors, wires all check good. Both coils have correct resistance on primary and secondary sides.

4) Both crankshaft sensors have correct resistance (540 ohms).

Assuming one bank of the V12 (right side?) is not running, I tried swapping some parts to isolate the issue.
- swapped MAFs and #1 above still holds true.
- swapped throttle bodies and #1 above still holds true.

My gut tells me this is a sensor somewhere, but what to check next?