Gnome Shell hangs when starting a new session

Hi,

I'm finding fairly often that gnome-shell hangs completely after logging in. This is happening 30-50% of the time when I start a new session. When it happens, the system is totally unresponsive to any user input. Keyboard, mouse -- nothing works. The only recourse is to hold down the power button until the machine powers off by force.

I never saw this problem with unity, nor with gnome classic.

$ uname -a
Linux hjh-e431 3.11.0-26-generic #45~precise1-Ubuntu SMP Tue Jul 15 04:02:35 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

$ gnome-shell --version
GNOME Shell 3.4.1

How should I go about troubleshooting this?

hjh