Before spending money on sensors and other crap. I would suggest that convert your car onto petrol. Check if the problem exists there as well.
Most probably what they have done is that there is a small L-key screw with which they adjust the accelerator of the vehicle. If your screw has not been adjusted properly then either you get high acceleration or low acceleration. This screw is located behind the air filter intake and it is not readily visible.
To check this the sure signs are that you convert your car onto petrol and watch the RPM in petrol. If there is a problem there then there will be problems even on CNG.
Whenever you go for CNG adjustments tell them not to play around with petrol settings. What most CNG guys do is that they increase the petrol RPM this results in a higher but moderate RPM when you switch back to CNG.