I have a zafira with factory fitted dual fuel. Runs perfect on petrol, When changed to gas either auto or manual idle goes haywire. Up amd down till eventually cuts out. Massive flat spot. Already changed gas filter. Code read, no codes present. Took to GM dealers ( gas division) for fault find. Not very helpful, told EGR or throttle body . So why not on petrol, no answer! Another symptom, car erratically changes back to petrol when cruising, could be 2 miles or 20 miles. But will manually change back but again for an untold mileage. All symptoms when engine hot or cold. Heeeeeeeeeeeeelp