Windows freezing

Hello All, I have recurring problem (not often) which started just recently. Thus far it appears random. The problem is that all my open windows freeze, can't be moved and do not respond to any input. I can open a terminal window (from icon) but cannot get input focus in this window I run gkrellm and that shows that the system is chugging along OK. If I use alt-F2 I can enter 'startx' and then all windows respond as they should. I am running an up to date Fedora 21 x86_64, and using Cinnamon. What might be the likely causes of this behaviour? Cheers, -- Regards, Terry Duell