GMC Repair: Missfire, auto part store, gmc sierra

Question
Why does my 99 gmc sierra 5.7 engine 4x4 misfires when put on reverse and sometimes at low idle then it goes away.

I already did tuneup cap fuel filter and map sensor.

Answer
I had the same problem. I scanned my codes and had two bad wires (new wires installed). At times, I have found new parts are bad out of the box. I would suggest, scanning. You can get a pocket scanner at any auto part store for around $30.00. Or you could test every wire. I would start there.