Automatic transmission slipping on Mercedes CLK

Sounds rough! I had a similar issue with my old Opel Corsa. The automatic gearbox was totally confused, shifting at random times. It turned out the hydraulic fluid was low, and the gearbox was overheating. I'd check the fluid level first. It's a cheap and easy thing to do, and might just solve your issues. Is the transmission performance consistently bad, or does it come and go?

Thanks for the suggestion! Where exactly is the hydraulic fluid reservoir located, and is it easy to check myself? Also, you mention overheating, is there a way to check that?

Unfortunately, I’m not sure where it's located on a Mercedes. On my Corsa, it was near the transmission itself, with a dipstick. Best to consult your car's manual or look online for a diagram. As for overheating I only saw a warning light. If the oil is very old and burnt, consider an oil replacement, but honestly, given all the symptoms, it's probably best to get it checked out by a professional.

Thanks for the advice! I took it to a workshop, and you were right, undervoltage in the vehicle caused the issues. They did an oil replacement and fixed the underlying electrical problem. Cost me around 240€, but now it's shifting like new.