Miss firing when mashing the gas slightly above 1500 RPM

1998 Chevy Lumina Car 3.1 It started when my son put a Amp. In his car put the hot wire on fuse box above the ECM & shorted the hot wire to the hood. He also added some fuel additives in the fuel too. Problem that accord when you would drive it 7 to 10 miles it would shut off. Then you could drive it 2 days then it would start the 7 to 10 mile mess up. When you tried to crank it when it shut off it would miss fire. After it cool down it would run but miss fire about ½ throttle let off gas it would level off. But when it was cold it would run good. IT started happing more often. It had codes on it for EGR & miss fire #5 cly.
I first started with changing out with new fuel filter, EGR valve, plugs & wires & cleaned mass air valve.
That got rid of the codes.
Now it cranks up but it miss fire when mashing the gas slightly above 1500 rpm.
I took the ECM out & check it by eye didn’t see anything wrong. Put it back same thing.
Please Help
Sincerely, James