I'm having a strange issue where my 98 e320 after idling an 1-2 hrs (cut that time in half every time if I have the interior or exterior lights on or I'm blasting my aftermarket stereo sound system) it begins to shake, idling roughly. Most cases I can turn the car off then back on and it's fine. With the case of the stereo and lights, it will idle rough then pretty quickly the engine will shut. After that it will start up but then almost immediately die or it will start up but will be idling a little rougher than before. The only way to get it out of what seems to be a 'mode' to me, is to drive it about a block, shut off the car and restart it, then everything is fine again. Both the alternator and battery are fine according to testing, and none of this happens while driving the vehicle with all lights on and system up to the max. Any possible clue to what may be causing this issue would be greatly appreciated.