Comment 53 for bug 194214

Umh, the matter is more complicated I fear. The first time it occured for me was after the Feisty->Gutsy upgrade. I've got xserver-xorg-core 2:1.3.0.0.dfsg-12ubuntu8.3 installed ever since.
The bug vanished for me after switching from Gutsy kernel 2.6.22-14-386 to Hardy Alpha2 kernel 2.6.24-2-386 (just the kernel, nothing else except nvidia-glx-legacy and libc6).
But maybe it is related to something that changed between the xserver versions you mentioned, anyway.