1997 Ford Crown Victoria Starting problem, bad electical co

I have an annoying problem that is getting worse. Every once in a while (getting more and more common) when I turn the key, all the power completely dies, radio, locks, lights, everything. Its not the battery or starter or alternator. If I disconnect the battery for a few minutes, it will usually start up again, although sometimes it has to sit for a while. Then it works fine for a few more months, then the whole thing will happen again. What could be wrong that just desconnecting the battery and then reconnecting it would fix (at least for a while)? Its frustrating because I've taken it in to the machanic and it worked fine for him, but a month later it happened again.

Thanks for the help