While using Xfce188.8.131.52 I use Alt+Tab constantly. I've noticed that when I use it, and there is only one window in the workspace that everything locks up and the CPU's "nice" time begins to work very hard.
It isn't until I press Alt+Tab again (sometimes a few times, it's truly random) that I am able to continue using Xfce.
If this isn't enough information then please email me and ask for more information.
It looks like a duplicate of #2161
#2161 is fixed and this one really sounbds like a duplicate. Please reopen if not fixed.
*** This bug has been marked as a duplicate of bug 2161 ***