I had a similar problem that occurred above 21K. Turned out to be in the
Mags. Internal arcing occurred. Pressurizing the Mags is required to prevent
this (for some reason pressurized air is a better insulator). Once the
arcing starts you have to replace some parts to get rid of it.

There are some small holes that are supposed to be open for drainage.
Apparently it is a well known and documented issue that these holes plug up.
If the Mags do not vent properly then moisture accumulates and provides a
path for arcing. We could not tell if this was my root cause.