@arqamshaikh
Seems issue of tuning in following manners:
1. Spark plugs could have carbon on tips
2. Carburetor mey kachra
3. Petrol filter mey kachra
Remedy: (Writing complete solution by assuming that your car is carby & having cng installed,DONT FORGET THAT 2K6 IS NOW DECADE OLD SO NOW U HAVE TO SPEND SOME MONEY)
1.Get your car fully serviced (include interior cleaning)
2.Get your empty petrol tank down,wash it from service station & reinstall it
3.Change Engine oil + oil filter + petrol filter + air filter
4.Tune your car both on petrol & cng & stick rpm needle to 1 or 1.1 K with A/c on & full head lights ON
5.Get good tuning of car with dynamo-meter
6.Clean your spark plugs & if change is required, then install new plugs
7.If you have budget, then also replace spark plugs coil too
8.Also get your cng cylinder clean & purely free from Oil
9.Get your cng kit overhaul or at least service of it.It might contain lots of oil inside it.