Thanks for looking at it, Randy.
It’s actually a Pixhawk 4, with one IMU mounted on a piece of hard foam, just like the Kakute, and the FC itself is mounted using a double layer of silicone sticky tape. And being a Pix4, all the wires are silicone AWG 28 - there’s no servo-style wires of thicker gauge running to it. And it’s a very compact copter, at 650 mm diagonal with 15 inch props, so the Z-axis vibes may be generated by prop airflow near the frame.
Now, after running a proper log browser, thanks to @ChrisOlson, some data became evident. Due to not wanting to squint while looking at the copter, my test flights were flown mostly facing north. And all the GPS glitches occur with the copter tilted to and accelerating north. No glitch was experienced flying E, W or S.
What happens is everytime the copter leans north, the GPS flickers by one or two sats, hdop flickers as well and the GPS VZ takes a beating, generating a glitch
The big yellow sinusoid is the latitude, those sharp blue/red spikes are numsats/hdop, the green/green/dark red curves are altitudes (desired, gps, baro) and in purple the misterious GPS VZ parameter