I recently built a robust lamp dimmer to drive an ignition coil. The dimmer
circuit uses both a triac and diac along with a capacitor, resistor, and
variable resistor. The dimmer feeds a 1 uF AC capacitor which in turn
drives the coil. I have the coil in a mineral oil bath to provide greater
output voltages without arcovers and I have sometimes been able to achieve
2.5" sparks to ground. However, I am finding output very inconsistent,
especially after the unit has been running for a minute or so. Both the
triac/diac are heatsinked, which helped a lot but I still can't achieve
consistent output. If I leave the unit off for a while and then come back
and turn on, sometimes I'll get the longer sparks and sometimes not. Any
idea what's going on? I've been thinking of switching to a NPO capacitor
and metal oxide resistor, but is this really necessary or is there something
else that could be done to improve output consistency?

Thanks,
Jack