Thanks fro the clarification bhai.
Well, I know that only two things are important when using cng/lpg. One is the gas itself, and the other is a good strong spark.
if these two are combines, the car should work well.
If other components in car are faulty, then the problem should exist with both petrol, and gas. As far as I was told, gas is less flammable as compared to the petrol, and that petrol will ignite with a lot weeker spark as compared to gas. Hence I was told that it is very likely that the problem is either the spark plugs themselves, or the HT leads (also called as ignition leads), or the distributor if you have one in your car (basically the spark producing components). As my car has electronic distributer, I was told that it can only be the spark plugs, or the HT leads. Since Ihad replaced the spark plugs only a few thousand miles ago, and they should last around 40000 miles according to Bosch, the problem was thought to be with the Ht leads. faulty HT leads will leak current hence weaken the current that gets to the spark plugs.
following this advice, I found the problem, and fixed it successfully. You can also check the same thing as it wont cost you anything to test, as below:
First of all, an easy check is to start the car in a dark place at night. look at where the HT leads go into the cylender onto the spark plugs. If you see the blue sparks jumping around then your leads are faulty, and should be replaced. they can go faulty due to their age etc too.
Failing that, take a bottle sprayer, like the ones barbers use to spray on your head when cutting your hair ...lol...
fill some water in it. start the car, preferably in a dark place at night, and spray SMALL quantities of this water on each HT lead one by one on each spark plug, and also at distibutor side where all leads go into the distributor. If the car engine stalls (starts missing, jerking etc) and/or you see blue sparks flying around then your HT leads are faulty.
Also if you havent replaced your spark plugs in a long time, then it might be a good idea to open each spark plug and have a look at it to see what state they are in. I am not sure how much they are for your car, but if you dont want to buy, then it might be worth borrowing from a known good car of same model, to try and see if the engine jerks still. But my money is still on your spark leads/ distributor.
If you do have a mechanical distributor, then it might be worth opening it, and seeing the state of the rotating arm and other connecting parts.
Word of WARNING: Do NOT spray WD40 or anything else flammable like that to clear the water you spray, as otherwise you may only see a Vitz Kebab (unless you carrya large fire exinguisher at all times), and a very well done one too. Just clean with dry cloth, and let it dry itself. be patient.
Let us know how you get on, Good luck, and Eid Mubarak.