Car suddenly cut off at red light

A couple of times in the last month the radio in my camry just stopped. Today, the radio stopped and the speakers went off. Lights then flashed and my car just cut off at a red light. It didn't turn over the first time, but did on the second.

I had the alternator and battery tested at auto zone and everything seems okay. What can be the possibilities? I don't want to be at the mercy of a mechanic who would perform expensive repairs and not solve the problem.