@ Everyone : I really know about this
if you have civic 2001-2005 ... On CNG and your car's RPM is not stable ?
Problem might be :
1) Most probably your advancer maybe water damaged! cuz they are really locacly made now adays! get them changed.
2) your CNG kit which is under your battery might be doing problem .. the button which is used to stable RPM mybe not working good.
3) and worst problem! but very rare, Your car's computer is distuirbed! i remember my cousion's 2002 EXi .. was not stable even on Petrol. the price of new computer of civic 2002 is 45,000RS ... but he managed to get it repaired or reset from a person in around 5000RS