Had a similar experience awhile ago on my 84 280CE, I was sitting in a drive thru line with lights, heater, stereo, heated rear window all on and my foot on the brake in drive. After sitting for a few minutes (ever notice McDonalds are getting slower) the ABS light pops on, but everytime I moved forward it went out, next the stereo volume started to get lower too. I finally figured it out, that all that equipment was too much draw for the alternator. After shutting down a few things it went out and has not been on since. I figure the heated rear window is the biggest offender, and I have read here somewhere that MB alternators are not always up to the job.

Hope it helps.