Ubuntu has been doing this for years. Usually I would just give it time and it would catch back up with itself (5 minutes or so). Recently, that hasn't been working. Essentially it's as if the entire desktop display freezes in place (except the mouse). Blinking things stop blinking and stay static. Things I hover over become unresponsive to the mouse. Back when it would catch up with itself, I could click something with the mouse and it would happen after the freeze stopped. Now it doesn't unfreeze and I have to switch to tty1 and issue sudo poweroff to fix the problem. This is especially problematic when trying to deal with word documents and browsing the web.
Specs: Ubuntu 14.04 X86_64 Inspiron 15R with i5 hyper-threaded processor 8 Gigs RAM
I use Firefox 34 and LibreOffice. Haven't installed any display drivers. Thought it was related to flash. Was mistaken. Switching displays and issuing sudo service lightdm restart seems to be my best option.