Ah ha! It was the oxygen sensor all along!
I swapped in a Bosch 13942 unit (1990 Ford Mustang 5.0), did some splicing and wah lah...

Kinda pissed my initial replacement only lasted 10K, that is the last time I buy a Denso unit.

Duty cycle now jumps between 45% and 55%ish.

Next Problem: Why aren't the aux fans coming on anymore...

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2006 BMW M5 "Heidi" @ 109,000 miles
2005 MBZ C55 AMG "Lorelai" @ 165,000 miles
1991 MBZ 300E "Benzachino II" @ 165,000 miles
1990 MBZ 500SL "Shoshanna" @ 118,000 miles

(On the hunt for a good used M103 engine as of 6/10/23, PM me if you have one to sell!)