New World NW60G Oven Fault ?

My New world oven kept going out and then trying to re-ignite called in an oven repairer who said the gas oven burner was at fault and needed replacing . He replaced the burner and then the grill kept going out and trying to re-ignite . He then replaced the thermostat,now everything lites and stays on but the oven doesnt get upto temperature , only way i can cook in it is to put everything on gas MK9 and double the cooking time ( he ended up giving me my money back and walked away scratching his head ) any ideas as to what could be the real fault ??