Silly question but did you try running it in the dark?
Bear with me here....
Could be a spark running away to earth.
Run it in the dark.... You'll see it better
I sorted my garage into 4 piles of stuff.
1. Stuff I`m keeping
2. Stuff I`m not getting rid of.
3. Stuff I might use..one day.
4. Stuff that might come in handy.