Afterfire at about 4k rpm

I finally finished my gt40p swap and the car pulls like a beast now. I am very happy with the project. Its a very fun street car now. The only issue its still having however is an afterfire out of the exhaust at about 4k rpm. Anyone have any ideas on what could be causing this? It also pops out of the exhaust when downshifting, but after reading some other threads I believe this is normal for the bbk high flow cats mid pipe.

Thanks!

I used the stock injectors and maf. I replaced the o-rings and pintle caps on the injectors. I also noticed while taking it for a drive just now that its idling a little rough. Not to bad, just a little bit however. Strange it didn't do this the first 4 or 5 times I started and ran it.