check the petrol soliniod valve. ( dono wat its called exactly, but its used when we switch to cng it blocks the petrol going to engine, its cng setup thing, will be of landi ranzo if its make of ur cng kit ) It might be faulty, mixing petrol, at low rpm moving or on sudden accelration its more prominent but once u start gaining the accelration, it never showed up and car accelerate smoothly till higher rpm. Can be checked like you remove the petrol pipe from it which is going to engine from that valve, if its giving petrol when car is start on cng, or giving very few petrol , its faulty, or either u can check like block this pipe and then drive for few mins and observe that problem persists or not ......
Check spark plugs, reduce there electrode gap for optimum current for cng. IT helps alot for cars running on cng ......
Get cng kit adjustment, cng flow screw so that it wont be running lean or rich.