My daughter's '98 Civic EX is giving me fits!

It has 92k miles. When driving (sometimes), the check engine light comes on, and the car starts bucking & chugging.

I changed the distibutor cap, rotor, wires, NGK plugs, fuel filter, front O2 sensor.

The parts store read out said:
- O2 sensor
- Miss-fire on 3 of the 4 cylinders.

I thought the O2 fixed it, she even said the car was running better than normal. Now today she called from college, and said the light came on, and it was screwing up.

Is it the injectors? Any advice would be great.