E500 - intermittent rough running

[donkmeister]

I've had a recent development with the E500 (2004 model, with the M113 V8 engine). Car has 105k miles on the clock, engine mounts were changed at 85k miles, spark plugs last changed around the same time. I haven't found any obvious leaks on the vacuum pipes at the front of the engine.

I've had two instances now where the engine has started running rough to the point that the car shakes a bit on idle (not that barely perceptible lope that M113s are known for on idle). There's no codes stored, no warning lights have come up. After the first instance I checked the engine mounts (replaced 2 years and 20k miles ago), these appear to be fine.

The first time, the weather was around zero degrees and the shakiness showed up once the engine got up to temperature. The engine was off for half an hour and then when restarted the shaking was there immediately.
The second time, the weather was around 10 degrees and the shakiness was only slight but again, showed up once the engine was up to temperature.
It's very intermittent, I have done several journeys without the shake occurring. On the occasions when the shake happened it was only at idle, and there was no apparent loss of power.

Any thoughts on what I should be looking at? I was wondering if its a coil starting to give up the ghost, but I've got no idea how I'd test that without getting new ones and seeing if the problem goes away.