i had a similar problem with my slk32. 3 times so far once at the starting line at the track. car layed down fluid and they held me at the line. shut the car off and it wouldnt start. then it did. twice in my garage.
found the negative cable is bolted to the front inner fender. the bolt was just loose enough to allow a ground loss. always checking it at the battery i never checked it at the other end. tightened it and no more power loss