This might be useful
http://community.wolfram.com/groups/-/m/t/291612
http://community.wolfram.com/groups/-/m/t/295310
There were people having problems when certain usb devices were connected. I would try to disconnect as many usb devices as possible.
Also under Preferences on the Tab interface you might want to disable things like dynamic and automatic etc. temporarily to figure out what the problem is.
Cheers,
Marco
