95 5.0 problem keeping volts

Hey I just bought a 95 5.0 the guy has trick flow intake 70mm throttle body pullys full length headers hpipe and flow masters. but I am having a problem I will be driving and have about 12 volts then the volt meter goes down to 8 or 9. I had the alternator and the battery checked out. I replaced the ignition modguel, coil, rotor, distibutor cap, voltage regulator, and it still dies out. When I changed out the coild she fired rite up and drove like a champ for about 30 min. then just died out all over again. Please help. I really dont know to much about the V-8s I had an 99 eclipse but wasnt fast enough.

check ur ground cable theres prbly a bad connection.