Ask your manufacturing engineers why they used a MAC97A6 at Q6. If they had used the MAC97A8, the latch wouldn't have been a problem. The wax motor isn't the problem, the triac is. (I know they will say that the A6 is the proper part) Put a Variac on the input line and crank the power up to 140 volts and just watch R11 and Q6 smoke. Then change Q6 to a MAC97A8 and watch the machine continue to run at 150 volts.
Contrary to popular belief our input power (the power grid in US maybe the best in the world, but it)is not perfect.