Bad Spark Plug??????

[USMC302]

On my 88GT, i just got done building it, which is most likely what is wrong with it. We built it from the piston rings up. Anyway, it has a light pop during idle like a miss but its not. It almost sounds like it is a valve not seating properly, they are remanufactured heads with a valve job, new springs, etc.. ANyway, under hard acceleration, it seems to miss badly but does get up and go, almost like it has 7 cylinders. The reason I think it is a spark plugs is because when getting the motor to run, it ran very lean, then very very rich, then we sand blasted the plugs to clean then and opened the gap to .060 I believe. There are also two different temp plugs in right now. The wires are 9mm, new, the cap and rotor have about 15K miles on them, and a MSD coil. What am I missing? What could it be? These plugs have about 500 miles on them, could one have gone bad already?