Since the car has been checked out by mechanics, I'll take a stab at an obscure cause for the probelm. A lot (most?) newer vehicles ... vehicles built since 2000 ... have a chip embedded in their keys that is necessary to start the car. If your mom has had a key cut by someone other than the dealer the key will open the doors, and it will even start the car, but the car will only run for a few seconds and stall. After a few attempts the ignition will go dead. If she hasn't had a third party cut a key, it's still possible the chip in the manufacturer's key is glitched and this would cause the same problems. I don't know if this is the problem, but figured I'd mention it as it kind of sounds like it's something obscure like this that's causing the trouble.
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