This started last weekend with what I thought was a fouled plug. It cleared up and off we went for an entire day of flying. Today we were headed NE to a new field with a restaurant we'd never been to with CAVU wx the whole way. As I did my runup, the left mag stumbled. Leaning out did no good so I called ground and asked to taxi back. In 11 years of ownership, I've never had a fouled plug. My mechanic pulled the lower plugs fed by the left mag and noted the bottom rear pilot's side plug looked different than the rest, like the cylinder wasn't getting spark. We replaced the plug with new, cleaned all the other bottom plugs - no joy, still a stumble. The more we tested, the more we realized that jug was cold. After removing the mag cover and pulling the leads he found our culprit: There was a tell-tale streak along the bottom of the housing for this lead and the lead itself had this same green corrosion on it which was cleaned up. Using several techniques we got it to look like this: . However, when we did a runup - confident we were finally going flying - I got the same stumble on the left mag. So now we're looking at rebuilding a mag or replacing a harness. Not sure which yet, but that streak tells me we might be looking at a micro-crack in the housing that would warrant a new mag. Ah well, best to get it figured out before flying season begins in earnest. Any thoughts about what caused that corrosion? My thought is it's gotta be a crack allowing moisture in??