2000 SLK 230 Problem

[mosey3]

From time to time ( it's becoming more frequent) when I attempt to start the car, when I turn the ignition key to the start position, nothing happens. All the lights on the instrument panel are on, as they should be, and the battery is not dead, car is in Park, and the steering wheel is not locked. Everything works including the headlights, power top, everything electrical except the Hazard Flasher. The last 2 times this has happened, the car did start, but a few short miles down the road, the engine shut off as if I had turned the keyto the off position. When I came to a stop and put it in Park, the above stated problem occured. Had to have the car towed home. The next morning, it may or may not turn the engine when you turn the key. Mercedes Dealer couldn't find problem. Any tech out there with a suggestion?