There was no receipt for any recent change to the plugs, leads, distributor etc., so it's anyone's guess!
I removed the cap just now. Here's a couple of photos. Should the inside of the cap be bone dry? Because it's not... spots of moisture on it.
Time to replace?
Edit: Anyone know a good parts supplier in North America? Just called my local Mercedes-Benz dealer and the cap is $89! Then my local regular parts supplier doesn't have the 230e listed :/