Screen goes green then freeze when xbmc started without "sudo"

Hello,

I have a strange (?) error regarding playing video in XBMC. I have installed XBMC on a Ubuntu 12.04 server mainly according to this guide , running on Nvidia restricted drivers (output: HDMI).

The problem: if I start XBMC from command line using "xinit xbmc-standalone" it works fine until I start a video - what happens is is that the screen goes green, I can hear the audio track for about 2-3 seconds then everything freeze (logging in to server via ssh I can see XBMC is keeping CPU busy to almost 100%). Only remedy at this point is of course to kill XBMC at command line.
The problem above is not encountered if I start XBMC with sudo, i.e. "sudo xinit xbmc-standalone", everything works fine then.

Same problem if I try to start xbmc at boot time with a script containing "su xbmc -c "startx /etc/X11/Xsession /usr/bin/xbmc-standalone" (in order to run as user "xbmc").

Am I missing something here? I am really struggeling to autostart XBMC at boot with user "xbmc" since XBMC-live startup script can not be installed (due to conflicting version I guess).

Thanks in advance for your attention