Electric shuts down
 

I have a 97 900SE with an intermittent problem in that I will turn the key to start and the car's entire electrical system shuts down. Everything goes dead. If I wait about 15 seconds, I can reactivate the electrical systems by pressing the key fob button a couple of times, but when I try to start the same problem recurs. To start, I need to disconnect and reconnect negative cable to battery and then re-set alarm, now everything works except radio won't accept my unlock code. I need to pull radio and disconnect and reconnect power cables and then the radio will accept the code. My mechanic thought problem was in the ignition switch and he replaced the switch but I had the problem again after. I am convinced something is going on with the anti theft system. Any thoughts??