My guess is that the coil was faulty. If you open one up you will find it has two coils of wire. The secondary coil is wound with very fine wire. If there is a break anywhere in that coil the brken ends will move apart when it gets hot but come back together as it cools. It doesn't need to move much. As the coil ages the continual expansion and contraction will eventually cause the breakage, so it is always a good move to change the coil when faced with a similar fault that you had.
