2006 Volvo S40 engine shuts off intermittently

Hi everyone. I posted this in the weekly car problem column but thought I should post here as well. We own a 2006 Volvo S40 2.4i that my daughter has with her down in Los Angeles. Occasionally, when she is driving at very low speeds, such as braking when approaching a stop sign or stop light, or when she is accelerating from a stop, the engine shuts off, forcing her to stop the vehicle, put the car in park, and restart the engine. Fortunately, the engine always restarts. Unfortunately, however, the problem only occurs intermittently. It may happen twice within an hour, and then not again for two or three weeks. As a result, the problem has not been duplicated by the dealer or any other repair shop. And, when plugged into the computer, none of the codes show up. So far the dealer has removed carbon from the throttle valve control unit and replaced the alternator. But the problem continues. Not sure what do to next as the dealer says he is baffled. Any thoughts?

Try driving it with just the ignition key in the ignition, and no key ring or anything else attached. If that helps, you probably need a new ignition switch. The stuff than hangs from the key swings around as you drive around, go around corners, stop, start, and that movement eventually will damage the ignition switch.

The key is a good idea. Also, check and make sure the battery is strapped down and cables are tight. Make sure the positve battery cable is not rubbing on anything and shorting out.