[FYI I no longer have access to my old email supporting a previous username...so this is my new username...I am not new to this forum]

Do any of the tech sages on this board have some insight into this observation:

My 1998 E300 TD (112,000 miles) has begun to idle somewhat roughly. It is not the motor mounts which were replaced at 80k or so. The engine sounds and FEELS like one cylinder is not firing...it is rough, not smooth. All other aspects of engine performance seem fine, including miles per gallon, acceleration, etc.

Here is the odd idle problem: if I turn on my headlights -- the idle becomes smooth -- perfectly smooth in fact. If I turn on only my dimmers, the idle seems to improve but only a little.

Why is the idle rough w/out headlights? Obviously the headlights present an increased load on the engine but --jeesh -- it would seem to be a pretty small load...