In a previous article, ECB said:
A fellow flyer said it could be a bad/intermittent ground between the
engine and the airframe.

Yup. This is what we found when it was happening to one of our planes:
http://xcski.com/gallery/rfc/38290SparkCbl1
http://xcski.com/gallery/rfc/38290SparkCbl2


--
Paul Tomblin http://xcski.com/blogs/pt/
"He's overweight, uninformed, and litigious. That's an American
hat-trick" - Lewis Black