Intermittent electric failure

I live in Orange County, California, and have a Saturn SW '97 with automatic transmission, power doors/windows, alarm and cruise control. 18 months ago the alternator and battery went bad and had to be replaced.

Recently, about 2 weeks ago, it would not start and lost all the radio settings (channels etc.). A few minutes later I was able to start it. This seemed like the same symptoms that happened 18 months ago. So last Friday I had the Saturn shop check the electric system, incl. battery, generator and all connections, and they found no problem.

Today (Monday) my car would not start - not even a sound when I turned the key. I tried to jump start it, no success. Lights, alarm - nothing worked. So I had it towed to that Saturn shop. Now the shop just called me and said they got my car, and it started without a problem. I'll leave it there until tomorrow when they'll check it again. They said if they can't make the problem happen, they can't fix it.

I know there is a problem even though it does not happen all the time. I really want it to be fixed so the car starts reliably anytime I need it.

Any guesses what could cause this kind of intermittent failure?