janry, Although not exactly the same, I also have a similar problem. For over 8 months I have been having the problem where the HDMI output of the WD box powers up set at 720p (I manually set it to 1080i, but about 50% of the time it comes up set at 720p which my TV does not support). Because of this problem I sometimes leave the WD box on for a long time.
With the WD box on and just sitting there, the color space changes from RGB low to RGB high all by itself. This has been happening with the past several firmware releases. However, after the new firmware release (1.04.31), besides having that problem, the color space now sometimes changes to YCbCr all by itself while the box is just sitting at the home screen.
I can not understand why the box is changing the resolution and color space values when I have already set them manually to what I want them to be. This problem should not be that hard to fix, and frankly, I can not understand why these problems have been there for over 8 months. When the user sets a resolution/frequency and color space value manually, it should stay that way!