Had the same problem with my 05 300C with < 2k miles when it happened. The air bag light in the dash would come on intermittenly along with a chime. Next time I started the car it would went away. Took it to the dealer and they reported a faulty air bag module. Ordered the part in and replaced in less than an hour. So far with 1000 miles since the repair, it's behaving with no problem.