It's just a hunch, but perhaps the problem originates from using the 64-bit version of Ubuntu.
I had a similar problem a while ago with an Ubuntu 9.04 Jaunty 64-bit install (it was a fresh installation, no upgrade, no beta, nothing special at all):
Every few hours the computer randomly crashed, even if the system was hardly consuming any resources, forcing me to reboot the computer by pressing the reset button.
I often found myself simply moving the mouse across the screen when suddenly everything froze.
I asked for help in an Ubuntu forum, posted syslogs, ran a memtest, checked the electrolytic capacitors inside the computer, to no avail.
I assumed some part of the hardware to be defective, but did not really know how to go on from there.
Eventually, when support for Jaunty ran out, I wanted to move to Lucid.
A friend of mine, whose Jaunty 32-bit system ran smoothly and who I had told about my trouble, suggested to go for the 32-bit version because he had a hunch that it was the 64-bit version that had caused me all this pain.
And what can I say? I have been running Lucid 32-bit for two years without a single crash, on the exact same hardware that Jaunty 64-bit had been on.
I never learned whether the problem did arise from the use of 64-bit, so I'm really just guessing here, but it might be worth giving it a try (even though it might be a bit frustrating to be sitting on 8 GB RAM and being able to use only 2 to 3).
These are my specs, by the way:
- Intel Core 2 Duo 3.16 GHz
- 4 GB RAM
- ATI Radeon HD 4850
- 500 GB HDD