S660 poor starting and stalling

It certainly sounds like a fuel-related problem. Given the symptoms you're describing – poor starting, increased fuel consumption, reduced power, and uneven idling – a likely culprit is indeed the fuel injection system. Have you noticed any black smoke from the exhaust or any unusual smells? Dirty injection nozzles could certainly cause these issues, affecting the fuel spray pattern and combustion efficiency.

No black smoke, thankfully, and no weird smells either. So, if it is the injection nozzles, is that something I can clean myself, or is it best left to a professional? And if it's a dirty injector, could that really cause the engine to stall?

While DIY cleaning kits exist, their effectiveness on modern, high-pressure injection systems can be questionable, and there's a risk of damaging the delicate components. Considering the engine stalling, it's best to avoid further damage: I would advise a professional diagnosis and cleaning, or even replacement, of the injectors. A workshop can perform a flow test to determine if the nozzles are functioning correctly. It sounds like your performance is being affected by clogging.

Thanks for the advice! I took it to a workshop, and they confirmed it was a defective injection nozzle. They replaced it, and it cost me about 515€. Runs like a dream now!