Man, this is a common problem with this M104 engine.
More often than not, it's a bad connection of one of the three coil wires to the boot resistor end.
Amazingly simple diagnosis. Surprises me that the dealer missed it.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".