how to diagnose a miss

My buddy has a mustang with a 302, edelbrock E street heads,F cam,24lb inj,255lph pump, FMU with disc for 24lb inj.,P600 with 8 in crank pulley and GT40 intake,msd,msd boost retard,timing set at 10* , retard at 0 for now.

He has devoloped a miss in it under boost. initially we thought it was his blow off valve because it was venting under light boost. so he checked the spring in the bv and found the spring wasnt bolted in at all,so we assumed it was just venting under boost and causing a rich problem and bogging and missing with too much fuel.
But now it isnt venting like it had and when he hits about 8 lbs of boost it still has a miss,he had set the boost retard for 12lbs of boost at 3 to pull timimg out under boost.He had to back the retard to 0 if we take out any more timing it misses really bad.

What other things do we need to look at to find the miss its causing? plugs are gapped at .035. we can't confirm that we have a leak somewhere in all the piping to the intercooler. with the belt to the blower off it runs perfect,pulls really hard.
So what gives?

Update on finding the miss.......my buddy said he fixed it.......he took the supercharger off and sold it.

well that's one way,
it might have been ignition related,
seeing as how it happened only under boost...