I've upgraded to Intrepid Ibex (Ubuntu 8.10), and, thankfully, the issue has disappeared. One other change that was made at the same time was to move from proprietary "fglrx" ATI driver to the open source "ati" ATI driver, so this may also be a factor.
A final update from me:
I've upgraded to Intrepid Ibex (Ubuntu 8.10), and, thankfully, the issue has disappeared. One other change that was made at the same time was to move from proprietary "fglrx" ATI driver to the open source "ati" ATI driver, so this may also be a factor.