Another weird issue with X5

[PEV]

Hi, I am another "lucky" owner of 2001 X5 3.0. Although I love it when it drives and the way it handles the road (I have manual stick), but unfortunately most of the time I spend fixing it... The last one is the weird one. I did find another post where someone had exactly the same issue back in 2011, but there was no resolution/follow-up, so I have no idea if it was fixed or how. Here is the story:
Started from it stalling on the road - was a traffic jam and everyone was moving really slow. My X5 started losing power and couple of times there was a loud bang from the engine (almost like shooting rifle sound). I stopped and could not start the engine after that. Called tow truck and brought it to the local BMW garage. It was closed and as I was waiting for someone to pick me up, I tried starting the car - it did!!! I could even move it to another parking spot. But still decided to leave it with the mechanic as it did not run properly with low torque.
Next day I visited them to learn that they ran diagnostics and discovered that the 2 sparks misfired. Suggested that I replace all 6 which I did (and all 6 plugs as well). Could drive a car for 5 minutes after that and than it lost power and stalled again. Next recommendation to replace the fuel filter. Done. Can start the engine, but no power and when I try accelerating - nothing. As if I do not press the pedal into the floor.. Removed the first plug and noticed that it is not "burned" - looks brand new. Disconnected the cable from that coil (#1) and behavior of the car did NOT change. That tells me that one of the (the only???) issues is that spark plug #1 does not ignite. Although coils are brand new, decided to switch 1 and 2,- the issue stayed on #1 (eliminates the coil). Did not try switching plugs.. could it be bad plug?
What can possibly cause this and how/what can I test now? I took my car away from that shop as no real help from them and doing it myself now.
Any help / advise would be greatly appreciated!!!
Thank you!