PixHack has unstable IMU when left to run, after a while the horizon flutters around, this is potentially very dangerous.
I have found a video on youtube that shows the exact problem where the HUD jumps around when the drone is just left standing on my desk with no motion.
Older PixHawk had this exact problem so i test all like this before I fly them,
I can understand your worries when you see that sort of behavior when testing out the pixhack before flying.
However that behavior is actually normal. Due to the nature of the AHRS (attitude and heading reference system) algorithm which fuses data from several sensors including the GPS, gyros, accelerometers and compass etc…
The cause for the jitter on the hud is caused by the GPS data which is usually very noisy since GPS signal is not 100% accurate. This effect is much more noticeable when you are indoors and your GPS position is jumping around by several meters which confuses the AHRS algorithm, and hence the hud jumps around abit. When you are outdoors and have a much more reliable GPS signal, your position will still jump around (as GPS signals are never 100% perfect) but this effect is much less noticeable. When you are in flight the IMU sensors will actually be changing and contributing data into the algorithm. There are other checks built into the ardupilot software which will check for bad IMU behavior and will display a message on your HUD/ not allow you to arm if there are any problems detected such as IMU inconsistency, or excessive IMU drift.
If you disable your GPS module you will see the HUD will stay stationary if your autopilot is not moving.
However with some of the later versions of Arduplane/Arducopter this problem has somewhat been minimized so if you do have this error from around Mid 2015 then it could be an issue that requires further investigation so it might be best to send it back for repairs/repacment.