From your description and the fact that the store said the charging system is ok I suspect that the problem is due to a bad connection in the main power buss. This is after the charging system. It sounds like a connection is heating up after time and the power is being blocked due to high resistance in the power buss line. Check the main panel under the hood and look for signs of heat damage to the wiring. The problem may also be with a power relay or the ignition switch connections so they should be checked if you don’t see any problem at the power distribution panel.