That's a good find, I guess igniter symptoms can occur at various revs. I noticed my idle was worse on humid days and had low rev issues with the igniter ( earth ) loose and rust on the bolt threads.
The solution I used was putting a copper earth wire directly to the ignitor from the battery the cars response was fantastic after doing this.
if i do connect the igniter earth like you did . do you think it will better ? any changes will occur ?