Seat Arona Diesel Engine Problems

That sounds like a troublesome misfire alright. Have you noticed any black smoke from the exhaust? When was the last time you replaced your fuel filter? Clogging can sometimes cause those symptoms you describe.

No black smoke, I haven't noticed any. And I haven't replaced the fuel filter since I got the vehicle. Could a simple filter replacement really cause all these issues?

A blocked fuel filter could contribute, yes, but with those kilometers and symptoms, it's more likely that one or more of your injectors are faulty. They're probably clogged or leaking, causing an incorrect fuel spray pattern and poor combustion. Given the emissions implications and potential for further damage, I suggest getting it checked by a professional as soon as possible.

Thanks for the input. Took it to a mechanic, and you were spot on. Defective injection nozzle. Cost me 515€ to repair, but at least it's running smoothly now.