As in the title, early in the morning, after just starting the engine, one or two cylinders are not working. I say not working because the idle is rough. After the engine warms up a little (towards the end of the blue zone on the gauge), it smoothes out. The temperatures during the night is about -20C, in the morning about -15, -16C.

What might be the culprit? I changed the sparks about two weeks ago to Beru Ultra. Beru website says these are exactly for my BMW.