i would say it is the fuel pressure regulator! i had a 04 that did the same thing. change it and all was good! just get ready..... I paid nearly 70$ for mine at the parts house. gm wanted over a hundred. go figure!
2004 pontiac gto 5.7, 6sp,ls7 clutch package, 160* t/s, comp cam package 238/248 @ 50 600/605 112lsa,399rwhp, 389 rwtq..