check for ignition timings. was there any work done in engine bay of your car? if not. Try filling CNG early in the morning 8 am or before and observe if the rpm fluctuation phenomena persist.
i changed the cng station for filling and my car average has come back to 100Km per 6.8 kgs and engine is smooth whereas from the regular station since 3 weeks car was weird with under 90 km per 6.8 kgs