1989 Mustang starting issue

Hey guys, so my truck is down and out right now, so I'm trying to get my mustang running properly so I have something to drive.

when I brought it back from vancouver, i got half way back and had to gas up, when I tried to start the car to leave, it had no power at all, roll started it and drove home with no issues, I thought the battery or the alternator was going bad, but I checked with my multimeter, and I've got 12 volts with the car off and 14 with it on, so all is good there.

Then we figured out that after the car runs for 10 or so mins and then is shut off, it won't start again, there is absolutely no power to anything in the car, but we tested the battery and it was still at a strong 12 volts.
Disconnected the battery for a min, then connected it again and the car started up with no problems.

So now I am wondering if it's possible that a starter relay or some kind of relay is overloading with power? I'm not sure where to look next.

Thanks