The 540 started running rough when stuck in traffic. You could feel the engine shaking and it felt like it was missing on a couple of cylinders or there was some fueling problem. I had a look a couple of days back and couln't find anything wrong, plugs all looked good and there wasn't any obvious which might be the cause. Had the hood up and gave the engine bay a quick clean and then found the cause...... a missing blade on the radiator fan. I didn't think think this was the rough running/ engine shake problem at first, I just thought ' oh great, something else to fix. Removed said part and problem cured.
Now to find a replacement fan.

Cheers