AHA! Thanks for the info, Trevor - something like what you're describing is probably the gremlin. I'll check all the wires as carefully as I can - I thought I did so when I put all the new coils on with the most recent igniter, but of course I could have missed something. I have a multimeter - what do you suggest I check? If there is a procedure already described in a previous post that you know of, just point me at it.
Given the time and money already spent on this, I was Googling around for alternatives, and found some guys who upgraded their Impreza-WRXs to new-fangled coil packs that have the igniters right on the coils. This takes the old-style igniter away, but requires some re-wiring of the harness to the new-style coil packs. I think I could figure out how to change their 4-cylinder wiring diagram into a 6-cylinder one. Has anyone tried this on an SVX? Here is the link:
https://forums.nasioc.com/forums/sho....php?t=1192028