I know this may sound bizzare but:

If you can take it for a ride in the dark with the side panels off you will see the arcing very easily if the insulation is breaking down, alternatively as a very short cure bind the HT cables with insulating tape, but the tape will also fail soon if the cable is breaking down.

Also had this happening with a Subaru when under full boost, more difficult to see in the dark tho'.....lol

Cheers