Honestly, I gotta push back a little on the "cheap HSS coated" thing. A lot of those Amazon packs are actually labeled as HSS but they're really just some mystery steel with a cheap TiN coating that does nothing. I've had way better luck grabbing a single, name brand 2 flute carbide end mill from a place like Lakeshore Carbide or similar for about $25. Yeah it's one tool instead of ten, but it'll cut clean through aluminum at full depth of cut without chattering or snapping, and it'll last through dozens of parts. That $120 pack is just false economy in my book.

The same thing happened to me on a King Air a few years back. The compass wouldn't align after power up and the AHRS would flicker in and out. I went down the whole checklist, swapped cards, reseated everything, even flew with the test equipment hooked up. Finally, I just decided to jiggle the whole harness near the magnetometer mount and the fault would come and go. Found a wire that was stripped a little too long inside the connector and a single strand was barely touching the pin next to it, causing a short that only showed up with vibration. A tiny piece of electrical tape fixed it. You really can't trust a connector until you pull the damn thing and look at every single pin with a magnifying glass, because a millimeter can be the difference between working and not.