Hello, I own a 2000 Kia Sephia and it has been running beautifully and getting great gas mileage until just recently. I will say after this problem has occured I have changed the spark plugs and bought a new coil pack for cylinders 2 & 3.
The car will run fine for a little while and then randomnly start misfiring extremely bad. I have to do all I can to keep it from stalling out. It will not misfire constantly but will misfire for 5-10 minutes and then straighten out for awhile. I am not sure what is going on but I have checked all vacuum lines and air intake and all of them seem to be in working order. I have scanned the ECU codes and they have come out P300 P302 P303. Basically the misfire codes for cylinders 2 and 3. Has anyone had experience with this before? Any suggestions would be GREATLY appreciated. I'm going crazy here.

Here are my guesses: First - do a compression check on all cylinders. If you have bad compression, you can have misfiring. Second - Of course have a look at the color of the plugs. All should be dry and a sort of beige color. Could it be bad spark plug wires? (I dont even know of Kia's have plug wires for 2 & 3.
Next; it could be a couple of fuel injectors that are partly clogged. The only method I know for diagnosis is to swap the cyl 1 & 4 injectors with the 2& 3 injectors, and see if the codes change to misfiring on 1 & 4. Then you could assume its an injector problem. I might just replace the injectors with 'rebuilts'. When injectors for my sister in-law's Toyota Avalon were about $100 locally, I got a rebuilt pair from a guy on ebay for about $40 (including the 'core' charge & shipping).
If the misfiring stays on cylinders #2 and #3, I would think it could be wiring harness of PCM problem.

I am having the same problem and the mechanic said that the plug wires are arcing?