Just had a new Thermo coupler installed, now I have a loud banging when the gas comes on, how do I fix?

My heater repair guy has been over many times in the past 5 days, each time he looks at it it works fine as soon as he leaves it starts acting up again. It has been banging like a bomb going off all you see from the heater when the gas comes on is a flash or flame and the burner cover flying across the room and hitting the dryer. Some of the parts that have been replaced is the Thermo coupler, gas tube to burners, new burners. Heater/AC unit was installed in the early 70's but repairman says there is nothing wrong with it. What else can I try to get it to quit?