Turn key then lose all power

[hath]

So I just picked up a junker 69' C10, and have been chasing the problems. I've replaced the starter solenoid and starter, installed a new main power wire from battery to inside fender terminal, bypassed the neutral safety switch, etc. Once I got to this point, I started the truck for about two seconds, it died, and had no more power anywhere, except from the battery to the starter. No power to the purple wire from the starter. If I let it sit for 10-15 minutes, it seems to reset itself and I'll get power again, but not enough to turn the engine over. It then loses power when I turn the key, and we're back at square one. It's a new battery, and I've replaced and/or tested the cables and grounds. Any ideas? I'm wondering what is resetting the power every so often. But I'm also wondering what is causing such a drastic draw on the system?