Anybody good at trouble shooting electrical?

Hey, posted a thread the other day about all kinds of problems. Well today I put a different PCM in the truck and would you believe it, the thing cranks over. I cranked it for about 5 or 6 seconds and stopped, then I go to crank it again and nothing. But the only light in the dash that stays on now is the check engine light, plus no more tbc fault. So I start trying to trouble shoot the starting circuit and come to find I'm getting power to the starter relay but from there it's not getting to the smaller wire going to the starter. So I change relays but nothing seems to work. So me being the genius that I am make a jumper to go from the hot coming from battery to the fuse box and touch it to the wire going down to the starter. Truck cranks over for like a second, then goes hay wire. Now you turn the key to the on position and all the lights flicker in the dash, almost acts like low voltage or bad connection, but batteries read 12 volts and I can't see any bad connections. I don't know, just getting frustrated. Anybody have any idea's. Could really use the help. Thanks