APM 2.8 with a slowly-spinning GPS map output to HUD

Hooked my copter up to Mission Planner this evening to work out some other problems, when I noticed something odd. Even with the copter remaining motionless on the workbench, the map on the GPS display in “Flight Data” mode continuously rotates clockwise around the point where GPS says the copter is located. It turns at about 4 degrees per second.

Additional info:

  1. I have not been able to get a good compass calibration on this copter since installing a new APM 2.8 FC.
  2. Yesterday the GPS was giving me no trouble at all. I have done nothing to the copter since then.
  3. All connections appear to be tight, and no items of any kind have been placed near the copter since yesterday.
  4. HDOP is ranging between 1.6 and 1.9, and a range of 8 - 10 satellites .

My setup:
Frame: Flamewheel 550 Hex
FC: APM 2.8
Firmware: A.C. 3.2.1
Motors: Turnigy Prop-drive-28 1200 KvA Brushless
Props: 9" Nylon
ESC: Turnigy Plush 30A
TX/RX: Turnigy 9x / Turnigy 9XBC V2
Compass: Ublox Neo 6-NM on a 6" pole
Battery: Turnigy 5000 mAH 30-40C 4S Li-Po