Since the second to latest kernel update of debian stable my screen flickered in normal xfce4 when there was movement on the screen. I thought it was a bug because i had nothing changed but the recent kernel update did not fix that. It took me some time but if you experience the same you may want to take a look at the dynamic power profiles which according to this
https://wiki.archlinux.org/index.php/ATI#Dynamic_frequency_switching
can lead to flickering when the card changes its frequency. I could not change the value with root so I changed the setting (=disabled) via this boot (grub2) parameter:
radeon.dpm=0This worked for me.