I have been a Spectra owner since 2003. My car was bought zero meter and now it has covered almost 170,000Km. I have had a number of minor issues. My car used to misfire up to 30Km and after that run just fine. Is your car fitted with factory CNG? If so then the problem MIGHT be due to bad solder joints on the curcuit board in a small control box located just above the clutch pedal inside the car. My car had a bad solder joint on one of the relay bases.
As I said that this might be the problem. Of course I may be totally wrong.