Vertical lines on 8.1 desktop

Yet another 8.1 problem is occurring. When I have been running for about an hour, vertical lines appear on the desktop. They are only on the desktop itself, not on any applications which are open. Sometimes I can get rid of them with "Refresh Desktop", other times I have to log out and back in again.. I have no screensaver enabled, nor is Power Control -> Energy enabled, although I use both in 8.0. Also I have tried replacing the 8.1 XF86 config file with the 8.0 one. No difference. I've not had this problem with any SuSE versions before, and I have not seen it mentioned on the list. Any ideas, please? Many thanks Keith