Help won't run right
 

I'm trying to fix a buddies camaro. Its a 2002 z/28.It has a weird runability issue that no one can figure out so far. It runs ok 3500 rpm and then it acts like it loses a few cylinders, runs like total shit and wont idle for a bit, and then goes back to running ok again.Anybody have any ideas how to fix it?

It's an '02 z/28 6 speed car. Ported throttle body that came off a goat, air box has been modified to move more air, bigger cam, ported heads, headers, no cats, computer tuned by speed inc. The car ran fine for quite some time after these mods, went 12.60s @ 114 mph on street tires. We've replaced the O2 censors, plugs, wires, map censor, maf censor, intake gaskets, fuel pump (fuel pressure was low went we tested it, was the first thing we checked), and tried an different computer and it did the same shit. It's had a map censor code, the code went away for a short time after replacing it . It was running correctly for about 10 minutes and then it went back to it's old shit. So there you have it, a recipe for making your hair fall out prematurely and money disappearing quickly! The weird thing is...the problem is completely consistant, the rpm at which things go south and the way it returns to "normal" never changes.