I have the following problem with Ubuntu 13.04: GUI randomly switches to console mode. If I suspend and wake up, it is in GUI again.
A related issue is when I switch to a different terminal (e.g. Ctrl+Alt+F1) and then switch back to tty7 (Ctrl+Alt+F7) - I see the same console I mentioned before (looks like a system log).
I tried all Ctrl+Alt+FN and Ctrl+Alt+Fn+FN, but it does not help. The only thing that helps is suspending (i.e. closing the lid) and waking up.