I'm not an electrical genius, but it sounds like a symptom of a problem with the alternator. It may not be quite ready to go south though, as the regulator, internal diodes, or brushes (if yours has them) may be causing the problem.
First thing to do is to check the voltage at the battery. Have someone rev the engine up to 1500 rpms or so and see if the voltage gets to about 13.5 volts or more. Also test with accessories on (A/C, headlights, radio, etc). It should be able to maintain about the same voltage, but no more than 14.
Also, a slipping belt could cause problems. Check that as well.
Just replaced the alternator on my 300E this weekend...it had a bad regulator and was pushing 17.5 volts!!! Wouldn't wish that DIY job on anybody, so I would look for all causes other than the alternator...good luck!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car