[CentOS] gnome / gdm mess

Hello,

My first attempt to install Gnome is not a bright success.

If I boot in runlevel 5, using gdm, I get the login screen, but after login I get sometimes only the root window (no icons, no toolbar, no menu, nothing but keyboard shortcuts), sometimes the icons are here but not the toolbars...

If I use startx from runlevel 3, everything is fine.

As I don't know Gnome, I have very probably messed something during install (CentOS-6, x86_64, using uptodate netinstall iso, starting from Minimal Desktop). But I have no clue about where to look, or how to debug it.

Can someone point me to some useful doc ?

P.S. I'm surprised to see gdm and X running on tty1 (in runlevel 5, not 3). Is it correct ?

Thanks for your help,