Could be something as simple as some slightly corroded electrical connections. Once you unplugged/plugged back in the sensor a couple times, it scratched the connecting surface enough to have contact. I'm sure you checked the contacts, but maybe try some 2000 sandpaper. There were also some reports concerning BMW electrical connectors that wrecked havoc due to the pins opening up too much, and making poor connection (on certain MAF's it was reported, and another part I can't remember right now). What people did, either changed the connector cartridge (pulling pins out) or twisting slightly the blade end that stabs in.

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Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.