Have you had the alternator checked to ensure it's cranking full power? And your battery tested, a 1v drop in battery performance (so 10-11v) will still usually start the car, but between a low voltage and low alternator performance it may not be handling the high load circumstances.
Could try driving for a week or two with the stereo turned off to see if it's something that's causing/contributing to your grief. Are you running an amplifier??
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