I recently went through the tasks associated with diagnosing and repairing an intermittent (not cylinder specific) misfire on my R53.

Bottom line up front - the wires to the ignition coil pack were shorting due to the insulation wearing off.

This was a hard one to find, because any shorting/sparking is happening inside of the loose heat-resistant cloth outer cover, within which the wires reside - you simply can't see it without luck, as the wires (in my case) looked perfect at the plug. I got really lucky and was able to see a spark from inside this loose insulated cover, slid back the cover, and viola! There's a 2 inch section of the green wire that is missing enough of the insulation to arc and short, thereby deliver too little juice to the coil pack.

I fixed this with electrical tape, will eventually put in a new set of wires. Since I repaired the short, the car has not thrown a code, my bank 1 O2 sensor code went away (I was dumping too much unburnt gas into the exhaust system, I guess - but not any more!) and the car's running like a scalded dog again.

So - wires and plugs and coil packs and injectors can be the culprit - but so can a short to the coil pack.

If you're having this issue, I hope this helps.

 

Do you recall what specific misfire code you were getting? My '05 R50 throws a P0300 a couple of times a week and misfires terribly while doing it. But as quickly as it sets in, it goes away and the car runs and drives fine.

 

Both of these. P0130 & P0300.

The 0130 is bank 1 O2 sensor, thrown because of unburned fuel being paased to it.