Unplug o2 sensor, it starts once?

[cp89ic]

Here's a brainteaser. For awhile, my 89 325ic would just quit, as if you turned a switch, but it always started right back up. Until now. After testing the usual suspects (fuel pump, air intake sensor, relays) And getting no fault codes from the Motronic, I unplugged the oxygen sensor and it started and ran perfectly! Tried again, and nothing. Plugged the sensor back in, nothing. Unplug it and it starts just once. Anybody have any idea as to what next? When it won't start there's no power to the fuel pump, power to the relays, but no power bridged between 85 and 86 on the fuel pump relay. Main relay and fuel pump relay have been replaced. I'm lost. Any help would be appreciated.