Shot in the dark - you might consider this post.

Intermittent miss in 1985 190E 2.3?

The business end of the spark plug wires on the 103 motors is subject to being easily damaged when removed. I don't care what anyone says, it helps to have the special MB tool used to pull the resistor tips off of the spark plugs.

You need to pull them STRAIGHT off and not rock back and forth. That approach damages them.

I'm not saying this happened, just saying it's a possibility.

Some people also use dielectric grease in order to facilitate future removal. Here's another interesting post that discusses this and plug removal in general. The guy who wrote this post uses an approach to plug removal that I feel is damaging to the interior of the resistor(my opinion). Then again, he mentions that the resistor tips can be a bear to get off if they've been on a long time, so it's sometimes catch-22.

How to change Spark Plugs? DIY?