Hello Peter:
This looks like a video driver/mesa problem. I bet you are using 3d
screensavers.
What video card and video driver/mesa versions have you installed?
Could you try to reproduce after having installed libgl1-mesa-glx-dbg and
your video card xorg driver -dbg packages?
Regards
--
Raúl Sánchez Siles
----->Proud Debian user<-----
Linux registered user #416098
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