Does ur car stalls on Idle at CNG only or on petrol too?
Now there are three chances, i would not waste ur time in asking whats urs so i'm explaining all three for u.
1- If the rpm is fine on petrol than ur throttle body and IACV is fine just spray some carb cleaner in TB if u want.
2- If rpm is not fine on petrol than open up and clean throttle body and clean IACV.
3- If rpm problem u're facing is on CNG ONLY, than its the time to refix the CNG settings which is usually required by an EFI car after sometime. If u're well aware of ur cars engine and cng kit then u can do it by urself, its not a big science