i suggest a compression test, else visit a computerised CNG tunning center, he'll let u know a rough idea about the status of cylinders, if u find something fishy there then must perform compression test. its better u do things quickly as time passes by, u might have to do a complete overhaul. but remember the first step, check tunning and status of plug wires, plugs as there might be incomplete combustion issue. get computerised tunning done for ur vehicle.