Take the cap off and check it yourself. You will readily see issues if the part has problems. Buy MB parts, not off brand parts from Pep Boys. I'm sure Phil has the correct part at MercedesShop.

If the cap and rotor is bad, usually you will see crusty corrosion at the dist cap contacts inside the cap, and the rotor contact as well.

Also check your coil wire and spark plug wires for wear, maybe pull some spark plugs to see if they are oil fouled.

If you replace the cap and rotor, make sure you pay close attention to the wire locations.