65 Barracuda daily driver

Check the bulkhead connector, especially the big wire from the alternator that goes thru the bulkhead to the amp gauge. It is probably getting hot, losing connection when the heat expands the terminal, and losing power. When it cools back off, the connection remakes itself and it will start again. I had a 73 Challenger that did that. I just cut the wires on either side of the bulkhead connector, pulled the burnt connectors out of the hole and ran a piece of 10-gauge wire thru the hole and spliced it with butt connectors until I could get a soldering gun to fix it. That stopped the problem, so hopefully it will for you. A little temporary repair like that won't hurt a thing since it's your daily driver. I know it's frustrating, so hang in there and good luck to you.