MissFiring

1990 integra Ls, jdm b16a1 motor swap, 72k on motor. This problem started when I went and got a new custom 2 1/4" exhaust piping done to my car, we also putted in a new exhaust resonator , but we reused the cat. Converter. After 4 days later the car been having some problem. So I took it to a shop he test drived and told me the timing belt was lose, so I replaced the timing belt and water pump. Took it for a driving test the car still missfire when I cruise down the street. Later that day I took it back and the guy told me he and no clue whats cause it to missfire so we thought it might be the oxygen sensor thats located near the cat. Converter, because when I got the exhaust welded on the guy who did the welding relocated my oxygen sensor to the header. So we instailed another oxygen sensor where the old one use to be at, got it wired up. Took it for a spin, it was running good for a lil while but soon as I enter the freeway it started again. This only happens when i'm cruising in low gear.I cannot fine out whats causing my car to missfire, when this happens it sound like the air that suppose to exit out the exhaust piping, feels like something blocking the exhaust piping. What causing this to happen I dont want to spend anymore money on something thats not gonna fix what causing it. So I hope u can help out