When the alternator isn't charging, and battery reserve begins to drop off, the car will begin to shut down non-essential systems in an effort to get you home or some place safe.
Take a look at the brushes (voltage regulator). Mounts to the back side of the alternator with two screws. On many models the brushes can be pulled/checked/replaced without having to remove the alternator.
Break out the DMM and see what your voltage is at the battery with the engine off, should be 12.9-13.2v or thereabout. Then check again with the engine running, should be 13.8-14v.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.