What could I look for that would cause the following problem? Some times when I'm driving, the car shuts off with no warning as if I turned off the key. When I try to restart, the key turns to the start position, but nothing happens. There are no keys hanging from the ignition remote key, and everything electrical works, including the instrument panel warning lights. It has a new alternator and the battery is fully charged. If this happens quickly, before the engine compartment gets warm, if I disconnect the battery and reconnect, the car will start. Is the compartment is warm, it will not turn the starter until the engine compartment has cooled to outside temperature Whatever the problem, it seems to be temperature related.

I am going to guess on this, seems like the reconnecting of the car battery clears some stored fault in the computer.
I had a Nissan that if engine temps rose too high the car would stall and not restart until cool. So maybe there is a temp sensor on your car around the radiator or heads that is defective.

But wait until a tech posts, nothing like the familiarty of working on a model to find a cure, and having the Mercedes diagnostic computer.

A relay refered to as the K40 powers up a variety of devices. If I were pursuing the stalling and the no start on that chassis I would see how the two tie together, the K40 may be the common bond.

the k 40 relay is a common problem with the slk//its a stand alone module and can be found under the pass side engine compartment under black cover