It sounds to me like your alternator and/or battery are right on the edge. Check out the battery and get a new alternator or replace the brushes in the existing one (easy fix).
The same thing happened to me. The tape player was slow for a couple of days, then the ABS light came on, then after a slow drive through traffic with the AC on, no start.
Brushes fixed the problem.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe