Any idea on an occasional "headlights go dim" all of a sudden thing?...

I have a 99 ML430 and I sometimes experience the headlight going dimm for about 1 sec, including the dash lights. It's like something is drawing lots of power all of a sudden and it causes the lights to flicker. It doesn't happen all the time...about 2-3 times having own the car for about 1 year.

This only happens when I'm at idle, brakes applied, in Drive gear and stopped in traffic.

It's not the aux fan cycling or a/c coming on. Doesn't seem to be anything electrical that I notice.

Anyone have any ideas? Someone suggested it might be an internal engine pump of some sort.

Thanks