95 GT running poorly
 

My nephew has a 95 GT with full mac exhaust, TFS street heat intake, E303 cam among other mods. Here's my problem, at full throttle when he gets to 4500rpm or so the engine sputters and runs pretty bad[:@]. If he backs off the throttle and drives "normal" it is fine. Got any Ideas? I had the O2 sensors go bad on my 87 and I thinks it acted the same way, but thats been years ago so I don't remember for sure. I have yet to check the plugs on it, I will do that next. Any help would be greatly appreciated!!!