Final update, the X is fixed and running like new again.

The cure was a new set of spark plugs ($345), but I also had a BMW inspection ($445), a coolant flush ($80), an hour and a half extra time for them to diagnose the issue ($150), and a brake flush ($100). Total pain: $1215

In the inspection they found some other issues. I'm wondering how many of these I can cure myself?
> High Pressure Power Steering line - they say it's leaking but the car is stored in a clean garage. Never a leak on the floor for me...
> Fuel intake flush/clean
> Front and outer CV boots - they say the CVs are "about to crack". How can you tell they are about to crack?
> Battery - I'm sure I can handle that one.
> Tail light assemblies are corroded.

Thank everyone for the pointers.