'00 C280 stalls/will not crank

[Jer]

Twice in the past 2 months my mother's '00 C280 would not start. The starter would not even operate. Eventually the starter would energize and the engine would start and run fine. The most recent occurrence was today when the engine just stopped in traffic and would not restart. She described it as just shutting off, not sputtering or shaking as if there was a fuel or ignition problem. She also said the power seat would not work a few minutes prior to this (not sure if they are related). I don't think this is related to the crank position sensor, as in this case the engine just turns off. I am led to believe the problem is in the electronic ignition key lock (not the electronic key itself). Any ideas?