When X keeps consistently restarting after logging in (you get to see the gnome menu bar), it’s probably the misdetection of X.org of the compositing abilities of the paralells graphics card.
Which means, you get logged in correctly and then gnome wants to enable desktop effects and boom – X Server Crash.
To fix it insert :
Section "Extensions"
Option "Composite" "Disable"
EndSection
into your /etc/X11/xorg.conf