GLK starting issues and rough running

Sounds like a frustrating problem. It could be the ignition switch, but those symptoms can point to a few things. Rough running makes me think about the ignition coil too. Have you noticed any pattern to when it happens? Like, does it happen more when it's cold or after driving for a while? Also, did you only check the fuses in the fuse box? Car electronics can be tricky.

Yeah, it is driving me nuts. When you say 'ignition coil', what exactly does that do? And which fuse should I be looking at specifically if it is the ignition switch? I really am not that good with car electronics...

The ignition coil provides the spark to ignite the fuel. If it's failing, you'd get a rough idle or stalling. As for the fuse, I couldn't tell you which one it would be specifically without looking at the manual, but since you are unsure, it might be time to visit a workshop. Intermittent problems like that are hard to diagnose remotely.

Thanks for the advice. I took it to a mechanic, and you were right. Turns out a fuse related to the ignition switch was defective. They swapped it out, and now it starts perfectly. Cost me 52€, which is way better than a new ignition switch!