Foxbody808

My car runs fine on idle, when I rev it on idle it's fine. But if I'm in drive and drive normally like not full throttle it's also fine. When I put the pedal to the floor the car sounds like its 2 stepping and sputters and the engine light flashes and it reads "multiple cylinder misfire". I've replaced plugs coils the ecu downpipe with the catalytic converter because the cat melted due to bad ecu and running rich. When I was driving it today I gunned it and the pipe I put in right after the cat after the 90 bend blew off. What could be causing this I dumped 800 into it and I only had it for 3 months now