nogas00civicdx

Ok so heres some background on the car. I recently bought a manual 2000 Civic DX with a B20Z2 swap (ECU P75). The previous exhaust setup was a DC Sports header with a rigged together stock exhaust. The catalytic converter wasn't connected to the rest of the exhaust so I had a new exhaust installed on the car (stock exhaust manifold with a test pipe and N1 style catback. The car was previously running fine besides the fact that it was obnoxiously loud. Ever since I had the exhaust installed the car seems to fall flat on its face in lower RPM's. I personally replaced the spark plugs (ngk coppers), NGK spark plugs, and distributor cap; the rotor button looked fine. I brought the car back to the shop that installed my exhaust to have them find the issue and it still does the same thing. They fixed some of the wiring on the o2 sensor, replaced the ICM, fuel pump, and fuel filter and its still doing it (NO CEL). I'm not sure what else it could be as I'm coming from the EVO world. Anybody have any idea what it could be?