i had this same problem my 600 holley carb.It' was not rebuilt, like i paid this guy to do.I fought this thing for a month.Everything was new,so i thought.I bought a new holley street avenger 670.Now it runs like a top.351c 4bbl eldelbrock torker intake,crane cam 272dur 524lift.Now i need to replace my FMX for a c6.Id say it's your carb.