LS no start Help

[paul mora]

Just completed by second LS conversion. Using a PSI harness for the first time. I got the entire package from a friend of a friend who I trust. Went to start it for the first time and it cranks fine got power everywhere but when I crank it I get a fluctuating voltage to the coils and other power systems. I gone over the wiring diagram and lit all seems right. Could the relays for the ignition and fuel pump send a signal that causes the voltage to shut down. The battery is strong and it show over twelve volts till I start cranking. I see the same voltage fluctuations. I have several ground to body frame etc. Any ideas where to start troubleshooting