Some time ago I noticed a strange behaviour on my computer running Ubuntu 12.04.
Here's a detailed description of the problem: I log into my account, work for some time, and then somebody else wants to use the computer so I switch users to the guest session. After some time I get back and also switch back to my account (without logout/login all the time).
Now my display always freezes upon actions with the Unity desktop. If I select an Application or scroll through the sidebar or even if I start a program and a window is created. Everytime my computer freezes for about ~10sec before continuing.
Also there are strange artefacts appearing (yellow or green blinking fields sidebar descriptions) and if I scroll through a page in firefox the upper "taskbar" is somehow "mirrored" into the page (but upside down).
I usually then have to restart lightdm to fix this problem, but it's a bit annoying to always restart lightdm.
How can I fix this behaviour? Thank you very much !