I have a 2005 R50 Cooper. A while back I replaced the O2 sensor after I got an error code, can't remember the number but it was "post cat mixture rich" or something. That fixed that problem.
Months later I get a P0301 Cylinder #1 misfire fault. The engine started running rough like it was on three cylinders.
I find a corroded terminal on the coil and so I replace the coil (MSD), wires, and plugs. Seemed okay for a while.
Then it came back. So I moved the #1 injector to #3. This did not help.
I put the old spark plugs back in, no joy.
I have gotten the fault and reset it MANY times.
The typical symptoms are:
The engine may be idling a bit rough, like a partial misfire.
It never occurs when I am on the gas or the highway and I commute 40 minutes each way everyday.
I will take my foot off the gas and the engine will stumble, the light will come on (blinking first), and the car will be running rough like it is running on three cylinders.
Here is an interesting bit, if I turn the car off and restart the problem goes away. Though the idle always seems a bit rougher than it should be.
Not sure what else to do besides try an ECM. ($$$$)