Such rarely occurring crashes are very hard to debug. It can be a hardware problem, driver (`flgrx`, as you've noticed) issue or even a kernel bug.
The easiest thing to do in your case is to change the video driver - of course that will only prevent further crashes only in case the driver was the problem source. Unfortunatelly, the right direction to go is not obvious: sometimes upgrading the driver can help, sometimes it can be better to rollback to a previous stable version.
You will have the highest chance of ruling out driver bug cause by switching to the open source alternative driver for ATI cards: either `radeon` or `radeonhd`.
Otherwise, if you really want to dig into the root of the problem and potentially help kernel/driver developers, you'll need some kernel debugging instrumentation. Have a look at LKCD, described in this article.