Comment # 9 on bug 781171 from Stefan Seyfried

Cleaning out old XFCE bug reports: does this still happen?
If so, could you provide a current messages file / journalctl -b output after
this happens, and the time when that happened (so that the events can be
correlated)

This does not look like an XFCE issue though, but rather a Kernel / Xorg
problem.