I have a Syncrowave 200 that is acting flakey on me. I am welding aluminum on AC 175A 7.5 balance. Sometimes the welder acts perfectly normal, other times I push the pedal, pre-gas starts, then I hear a hum but no spark. I let off the peddle and get post flow.

Why no spark?

What information have I left out?
Thanks for your help.
Chuck