Mini Countryman rough idle and stalling issues

Rough running and stalling on a Countryman sounds familiar. I had similar issues. First, have you checked for any engine misfire codes? Fuel delivery problems often trigger those. Could be a faulty fuel pump relay. Also, when was the last time you inspected your fuel injector connections?

Misfire codes, yes. How would I check the fuel injector connections?

Yes, those misfire codes point towards fuel starvation. As I recall, a defective fuel pressure sensor could be the culprit. It's plausible a faulty sensor is disrupting the fuel pump and affecting fuel delivery. Inspect the sensor's cable and plug connection. If that looks good, I'd suggest a visit to a workshop. They can properly diagnose the fuel system.

It was the fuel pressure sensor, just like you suspected. Defective cable connection. Replaced it for 95€. Runs like new again. Thanks for the help.