I own many different chevys and never experianced this until my first corvette a 1975 and low and behold the electrical gremlin. I drovecar hard  for 30 min. I hit power window switch and zilch , I lost 100% all electrical power to everthing. Opened the hood and started pulling on wires checkin connections found an orange wire with a replaceable fuse laying on the exhaust and not connected to any thing by the starter side ( still not sure were this goes ) I pulled it a side and pulled fuse then wife told me dome lights were on so got in hit the key and drove off. We were 20 min down the road when I kicked high beems on and zilch nothin at all. I checked everything and nothing worked. I checked connection on the battery and seemed good. We were standing out side the car waiting for a tow when everthing came back on 10 min later. I need to get TA out of the garage to look under vette but somethin aint right. Since this day I did remove entire dash to check wires around headlight switch, dimmer switch and window switch, and found two grounds were melted away, one to the door frame other to speedo bezle. Fixed those and I have driven it around town, but not on hi-way yet. Had a new starter and alt when I bought the car month ago. Was told wipers and AC did not work. I can tell that It needs some wires fixed under the hood. I.E. Cut wires to the AC box resistor were wired to by pass this. Wires spiced in for the new choke and the idle. Still need to understand were the orange wire should be  hooked up to, if anything. Need to check ground and fusible links. Is this engine wire harness the same as F Body's, were it is bolted with a plug to the fuse panel?  What is the orange wire in question

It sounds like a circuit breaker cools off and resets itself.  You must have too heavy a load on the system somewhere.  That is one of those tough problems to find.  If you have an inductive ammeter, you can check various circuits for load.

Larry