Seat Mii running lean

Whoa, lean running is never good! If your Mii is jerking and the CEL's on, it definitely sounds like a fuel issue. Could be a dirty or failing fuel injector, like you suspect. But before you go swapping parts, have you checked your mass air flow sensor? A bad MAF can mess with the air-fuel ratio too. Also, what about the oxygen sensor after the catalytic converter? Is that working properly?

That's helpful. How would a faulty oxygen sensor cause the engine to run lean? I thought that was only there to check if the catalytic converter is working.

Okay, an O2 sensor after the cat can affect the air-fuel ratio because the ECU uses its readings to fine-tune the fuel mixture. If it's giving incorrect data, the ECU might think the engine is running rich and compensate by reducing fuel, causing it to run lean. But since you've got multiple symptoms pointing towards fuel delivery, the injector is the most likely culprit. I would take it to a shop and have them check the fuel pressure and injector spray pattern.

Thanks for your help! You were spot on. I took it to a mechanic, and after checking the fuel injectors, they confirmed that one was faulty. Had all of them replaced. Cost me 515€, but now the car runs like new again!