X freezes sometimes, after closing the lid on an asus-k52f laptop with ubuntu 12.04. and i have to reboot it, wich is kinda anoying.
What do you have your laptop set to do when you close the lid?
There's been a known issue with the stock Ubuntu kernel where the computer locks up when the screen dims or shuts off. The recommendation has been to upgrade the kernel to 3.4 if you want to be able to dim/turn off your screen. If you don't care, then you can simply disable the settings to turn off your screen in the power management and display settings areas.