I reset the codes and they haven't come back yet (knock on wood).
I didn't know about the "has to happen twice before it triggers the SES/CEL/MIL" rule, interesting...

The current spark plugs have ~30k miles on them, so I'll replace them as soon as the weather gets better - it'll be interesting to see what they look like (maybe that gives me a hint...)

Thanks SarTech & Qsilver7!

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2005 X5 4.8is, ~64,000 miles, Le Mans Blue