Sudden loss of electrical power

I am the original owner of a 2000 Toyota Avalon. It has been well maintained and has 170,000 miles; I am hoping to make 240,000. On two occasions last week, I drove somewhere. When I returned, the engine was still warm when I started the car. The engine started and was running when suddenly I lost all electric power, (lights, gauges, radio, etc.), the engine continued to run. Within a minute, all electrical power returned, (similar to a computer re-booting). When I took the car to a Toyota Dealership, the tech told me the car ran fine and didn’t have any problem. He had no idea what the problem was and didn’t know where to begin looking. His only advice was for me to wait until it fails completely, and bring it back in. Any idea what may be the cause of the sudden loss of electrical power?

A good idea could come from a close exam of the electrical schematic,some people on the Forum seem to have a ability to get these,I am to lazy. (Christmas vacation from school)Really look at the schematic and identify a point where a failure would cause the symptons you describe.