I had a similar problem with my 420 SEL some years ago, and it was driving me crazy. It turned out to be a loose nut holding a ground wire. Sometimes it would vibrate loose and the car would loose power because of loss of electicity. Of course when it was just idling, there wouldn't be any vibration to make the ground loose contact.
If you haven't found the problem, you might let the car idle and shake the all the wires (except the sparkplug wires) and see if you can replicate the problem.

Good luck