So in response to my own thread - and I may be jinxing it here - but after pulling the plugs yesterday, I cleared the codes. It's been 24 hrs and 30 miles and the light hasn't returned. I did not torque the plugs down originally when I replaced them. I didn't torque them down this time but did tighten them more than I tightened them originally. I'm going to ride this wave out and see if the light stays off. If so, I will torque them all down properly. I think with them being loose, especially one or more in cylinder 4-6, it was letting air in past the threads.

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2006 X5 3.0L - Schwarz Black/Truffle Brown
Stock
But I keep it clean