many crashes of X server, what should I investigate?

[Frédéric Bron]

I run F17 with regular updates.
I have many crashes of the X server:
- freezing when switching between user
- freezing when starting a session
Many times, Ctrl+Alt+Backspace does not work, also Ctrl+Alt+F[1-9]
does not work either so that I cannot even start a terminal session to
see what happens. I find it difficult to justify my choice of linux
over windows in front of the familiy...
I would like to investigate but I do not know what to look at. Could
you help? Are there some logs that I can look at after the crash?

I used to run GDM + Gnome sessions. I changed to KDM + Kde but still
have issues.

Thanks for helping,

Regards,

Frédéric