My Ubuntu MATE 15.1o after a period of use will suddenly start showing Hi CPU usage as shown in SYSTEM Monitor. (Switching between 100% on each of 2 cores)
Looking at Process the top user is XOrg lightDM service.
If I use the MATE TWEAK Windows Manager and switch the Window Manager to another setting it stops the Hi usage. It does not matter which one I switch to MARCO/Metacity/Mutter/Compiz just the fact that I switch to a different one causes the Hi CPU usage to drop to normal.