Electrical problem on 2001 M5

Hi - new to this board, but looks like I might get useful assistance so here goes. I have an (very) intermittent problem where I lose all power briefly (ranges from flickering of the BMW monitor to complete engine death). Usually happens while driving along happily, often at high revs, once idling in traffic, when it stalled the engine. Car runs very happily the rest of the time. I had a friendly mechanic check the logged error messages, and it had only one, which was '54-main relay voltage'. He concluded that it's either the main relay or a fault with the ignition switch. I'm gathering as much info as I can before resorting to a main dealer, so can anyone verify this as a likely cause, and perhaps tell me where the main relay is so I can have a quick inspection of it. I am a little wary of dealers since I have fitted a laptop in the boot for MP3 and video playing, and I feel this may attract their attention as the likely cause, although I am ~90% certain it's not.

Any help gratefully received.

Thanks