X restarts sporadically when browsing

We have bought a Dell Mini 12 (poulsbo chipset) shipped with Ubuntu Hardy 8.04. We started to use it 2 weeks ago. Since then the X restarted suddenly twice, both times we were using firefox and browsing. The second time (it was yesterday) it crashed without any input action (scrolling the touchpad or typing).
I've checked Xorg.0.log and it showed this:
***********
Fatal server error:
Fatal video memory manager error "Device or resource busy".
        Check kernel logs for more info.
        Detected in file Xpsb_bufmgr.c, line 391, function
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this is a copy-paste from https://bugs.launchpad.net/moblin-other/+bug/193556/comments/3 because i did not save mine unfortunately, but the file name and the error message in quotes is the same (if it happens next time i will paste mine). And please take a look at this answer: https://bugs.launchpad.net/moblin-other/+bug/193556/comments/9 - this is pretty much the same situation as mine.

This source file is in xorg-modules-xpsb, am i right? My system is up-to-date and here are the relevant package versions:
libgl1-mesa-dri-psb 0.24+repack+0044-0netbook1
psb-video 0.30+0044-0netbook1
xorg-modules-xpsb 0.18+0044-0netbook1
xserver-xorg-video-psb 0.30.0+repack-0netbook2