99 Suburban Electrical Problem

I have a strange electrical problem with a 1999 Suburban. For no apparent reason, the vehicle will lose all electrical power - and I mean everything. Power locks won't work, clock resets once power returns. If you let it sit, the power will eventually return - sometimes after a few minutes, sometimes 8 hours later. I've check the battery connections, every ground wire I can find and wiggle every wire under the hood to try to replicate the problem, but haven't found a cause yet. I talked to a service tech at my local Chevy dealer and he said it sounds like a bad ignition switch. This doesn't make sense to me. Would a bad ignition switch cut power to everything, including the power door locks? Anyone ever have this problem before? Thanks.