Car shuts off while driving!

[lsmalley]

every once in a while when i am driving my car loses all power and all the lights on the dash turn on and i have to turn the key off and then restart the car. 3this isn't even limited to stopping at a stop light or stop sign, but while my foot is on the gas! alternator was replaced 6/19/08, but was a bad alternator installed by mechanic and he installed a new one on 12/29/08. any suggestions guys? thanks.