I always go for the simplest things first. I guess from your first post that the only check of your battery is to determine if there is sufficient acid in it. It is not clear that you have ever changed the battery or checked it for output both with the engine running and with the engine off. In my experience, these cars are really sensitive to a low or worn out battery. When you are sitting at a stop light and idling, it may well be that the alternator is not able to charge the battery sufficiently to keep it running. Before doing anything esoteric, I would want to be satisfied that both the alternator and the battery are putting out the necessary voltage.
This comment is to be valued at the going rate of $.02. Good luck!
Telechronos
1999 S500 Grand Edition (mileage 38,063)
1984 380SL
1987 BMW L7
1969 Factory Stick Pontiac Grand Prix SJ
1955 MG TF1500