Facing exactly the same problem from last two weeks ... Mechanic replaced the Plug Wires, Spark Plugs, Coil, Adjusted Tappets, Relay in CNG Kit, Washed Distributor and did whatever he could do but no luck. Went to electrician, he said everything is fine from Wiring point of view.. then he suggested to go to CNG Service Station and get your CNG Kit serviced...
Though car isn't jerking like before but when I leave the car started, I notice minor jerks frequently like missing ..