Hy Village_Idiot, is the ignition switch new or an old one, it could be that the switch contacts are not passing as much current as they should when the switch warms up. Also how hot is the ballast resistor getting, the hotter it gets the less the current flows, and restarting bypasses the ballast resistor. Is the coil mounted in a bracket that will allow it to disipate heat, the hotter the coil gets the weaker the spark is, please check your voltages at the ballast resistor input and output. A bad coil can do what you are describing, and a bad condensor can cause this as well, however they don't usually recover immediately, but a switch with bad contacts will sometimes recover immediately, because you have cycled it and the contact may be better, hope that helps, and good luck.
