Magnetometer yaw resolution only +/- 10-20 degrees

[HSPalm]

Hi,
first I had a multiwii standard fc (mega328p) but I lost the whole quad and bought a second one with bigger and better Crius All-In-One V1.1 (mega2560) fc.

Same issue on both, if it is an issue... The YAW value under magnetometer only differs between 160-180 degrees. Sometimes it's all the way down to 10-20 degrees but the resolution stays the same. I have read and tested many ways to calibrate the magnetometer, but same result. Sensor on both fc are HMC5883L.

The real issue here is yaw drift while flying LEVEL mode. Both with and without MAG mode on, it can sit perfectly still for some seconds and suddenly start to yaw (no wind) to one direction, sometimes it even stops at 90 degrees or thereabout.

I am certain I don't have any other flight modes activated other than LEVEL and sometimes MAG.

[HSPalm]

Can anyone be so kind and observe the YAW values of the magnetometer, and post here about it?

What I'm suspecting is that since I'm far north (Norway) it's mostly the pitch and roll values that complete to the total YAW value? But I really have no clue.