I had a similar problem with my 82 380SEL Mercedes......My problem and yours probably arent related however I noticed my car would not start after about three days of sitting idle. My car also had a viper anti theft system connected to the electrical system.......Turned out that this anti theft system was the culprit.....I disconnected all leads going to the viper box and problem was solved. I dunno if it was simply connected wrong or what, but anyway this fixed my problem.