2000 Kia Spectra

I have a 2000 Kia Spectra. The problem is when I accelerate the car chugs and the check engine light comes on. sometimes at idle it is very rough. When I read the code it is p301 misfire on cylinder one. I have changed the plugs, wires, injecter, and the catalytic converter. I dont know what else to change or where to go from here. I am getting a P303 code on my 03 kia spectra. When i start car or when it is in idle, it runs great. When I put a load on the engine, I get a misfire and check engine light flashes. Replaced Plugs, Ignition coil, wires and cleaned mass air flow sensor. Cleared the codes and took it for a drive. after a couple minutes on the highway, the symptoms came back. Checked the codes and P303 again. Checked volts going to injectors, checked resistance on injectors, fuel pressure is fine. I even swapped the engine coils but still have same problem. I didn't check the computer.