[excitement302]

Hi, I'm new to this site and I just bought a 5 speed 89 Mustang GT. The car seems to run fine and strong doing normal driving but everytime when I try to step on it under hard acceleration (above 4000 rpm), the exhaust starts to make some sputter noise, the engine hesitated and lost power instantly. It seems like a backfire from the exhaust. If not, What can it be? The car have the following modifications: 65mm throttle body, 70 mm mass air, mac cold air, pulleys, 1 5/8 shorty headers, mac pro chamber, 2 1/2 mac catback and 3.73 gears. Any help or advices will be greatly appreciated.