hmmm, i was thinking it may have related to the 2.6.17 kernel series. As there have been various strange issues cropping up with it. 220.127.116.11 is listed as 'stable' on kernel.org. But if the distro are using prior kernels then that would count that out.
I can remember an intermittant sound card problem i had a few years ago that traced down to oxid on the connectors. isopro alcohol and cotton buds solved that. I can also remember having problems, such as cd drive locking, disk freezing that stoped on uping the cpu from a generic 400w to 480 TruePower.
You also mention it will freeze up with out having much running, so i guess that counts out the cpu.
The only other thing would be something to do with your board/bios ... power management, irq management ...
If the ram had an issue, why didn't memtest fail ....
What is there about firefox that seems to bring it on ...
What I didn't mention before was that the system logs (in Ubuntu at least) didn't give any clue.
cryptic buggeres arn't they
. Hope it doesn't get much more obscue.