HELP. NEW MOTOR

Just installed a new motor, top and bottom end. Got everything in fine. Got the timing pretty close just by idle, and easy start up. Truck starts up and idles great for 30 min, after that it dies. If I go to try to start it back it acts like the battery doesn't have juice but it does. I was told maybe it had to do with vacuum assist but I know nothing about this. Any help I can get I'll take. And i can give any answers I can.
Thanks guys