An issue I have had is that XBMC on ATV2 was very unstable, for example a movie scan, a period of inactivity or adding an SMB folder would crash it to FrontRow. However it seems to be an issue with having ATV Flash Black and XBMC together. Follow the guide below to see if it helps you.
1. Power on ATV device to FrontRow (home screen)
2. Select Maintence
3. Scroll down to Media Player
4. Select and Remove Media Player
4. Reboot ATV2
Hopefully this will fix major instability issues. If it has or hasn’t, please comment and I will try to help.