Thanks for replying Josef.
Yes I am using the graphic version. My system is on 24/7. I occasionally look at the graphics for a WU for a few minutes, sometimes by clicking the "Show Graphics" button in Boinc Manager, sometimes by looking at the screensaver.
The error has only happened once in at least 20 WUs, and it did occur shortly after I pressed "Show Graphics" on a freshly started WU.