Well it could be be due to the buggy intel drivers in Jaunty.
I used to get crashes similar to yours when I had just 512MB RAM. Always used to be Firefox, sometimes with Flash; the whole system froze. I had to do a hard reboot.
I upgraded to 2GB RAM and now I have no problems.But at that time I was running Fiesty, Hardy, Intrepid etc based distros including Mint. Now I am on Gloria. But even when I had that problem only Ubuntu based distros caused it. Puppy, Pardus and even PC-BSD were fine. It used to happen more with Gnome than KDE.
But I haven't had that problem for a really long time with Jaunty or Gloria.