The best advise can be given after physically inspecting the car, anyhow as per my meagre knowledge the following could be the cause of you problem
-- You getting problem on CNG and that too after a couple of days (i presume)
-- Your kit has oil, the idle control is not controlling the idle perfectly
-- The gas is impure it requires a different mixture initially and when impurities are used it needs re-adj of idle setting.
-- You system is taking in additional air (not a very likely reason)
There might be something faulty related to cng kit but i don't have much knowledge in this area
If you are not facing any problems running your car on petrol then i guess i've completed the list