[hharney]

From my IA:

I’d send the suspect magneto to Wayland Bruce at Magneto ER (easier said than done on the front engine of a T337).

A few years ago I had a similar issue with the rear mags on a P337G and Magneto ER was able to pinpoint the problem and fix it quickly and inexpensively. The mags have been running for a few hundred hours with no more issues.

If the magneto itself checks good then the pressurized air line is the next suspect. Is that magneto getting hit with oil residue from the pressurized air? Make sure there’s a good filter installed in-line (RAM Aircraft sells them).

A lot of shops have a high-tension ignition lead tester. I’d doubt that the ignition harness is a cause, but if a tester is available, it’s a very quick and easy test to rule out the possibility.