Mazda 626 V6 2002

I have Mazda 626 V6, 2002 model with 70K miles approx. I had changed the plugs that I bought in September (about 65 K Miles) from AutoZone. Then in December my car start jerking and shocking while accelerating and check engine light came on. So took the car to the dealer. They told me that I need NGK plugs, wires and Fuel injection server. I was told the total damage would be 700$. Instead of that I decided it to do it myself.

For half the price, I bought the same NGK plugs and wires from a online Mazda Dealer and had the local shop put them on for 30$. I also bought the fuel system cleaner in my car (6$).

The check engine light is gone because the mechanic had reset the battery. The car does not jerk and give shock at least not that much. But it still is misfiring a bit. While accelerating the driver can feel it.

The question is should I get the fuel injection service? What is involved in fuel injection service?

Any other thoughts what could be wrong with car OR what could be the cause of misfire?