My 1998 Lumina has code PO303

My 98 Lumina according to the computer has a missfire in cylinder #3 which is code PO303. what is happening is that when i am driving the car starts to sputter and loose power like it's going to quit. but after about five minutes the service engine soon light starts to blink and then stays on permenantly.the car will then smooth itself out driving normally. i have taken it to Auto Zone three times to check and reset the code. anyone know what could trigger a missfire in cylinder 3 .

how are your plugs and wires? if you can remove the plug for cylinder 3 check it out yourself or take it to autozone and have if looked at. The plug may have incorrect gaping or even bridging.

how are your plugs and wires? if you can remove the plug for cylinder 3 check it out yourself or take it to autozone and have if looked at. The plug may have incorrect gaping or even bridging.
The wires and plugs were changed at the same time about a year ago. i watched the mechanic install them one thing i noticed he didn't do was check the gap of the plugs. he siad they didn't need checking because the GM plugs are already pre-gaped. also i had a bad fuel rail a year ago and i replaced the rail from one i got from the junk yard. the rail had all the injectors on it so i told the mechinic to replace the whole piece with the injectors already on it. is it possible # 3 injector could be bad. my problem started with the PO303 A code after i replaced the wires plugs and fuel rail.

Definately possible - check the spark plug for injector #3, if it is dirty or foul, check the resistance of the injector against the rest of them. I'm not sure of the resistance value of a '98, but your injector values should all be fairly close. If #3 is not, I'd replace it.