Question on power for a few minutes then none

I'm helping a fellow out getting his 70 slant 6 Duster back to driving condition. He was diagnosed with Liver cancer and has less than 2 years to live. He wants to drive his Duster again that hasn't been road worthy in about a year and a half. I went to his house and fired up the car and it ran pretty well for about 5 minutes, then it died. Nothing in the car had power. lights ,dash, ignition , I mean stone dead. After about 5 minutes we had interior power again and it ran again for about one minute, then dead again. Yesterday before I had his Duster towed to my house. His grandson fired it up and the same thing happened again to him. While I was there I checked the obvious battery connections, Power wire at the relay but no luck. I'm going to put it on the lift and start going through the wiring.
But I'm thinking that more than likely he has a bad amp meter? Any other thoughts? This is a pro bono effort on my part. The man just wants to enjoy his Mopar while he can. It has a bad bearing in the 7 1/4 rear which is what I was going to fix for him. I have a wiring diagram for reference.