[PROBLEM]
I've installed Ubuntu Dapper Drake (6.06) on an IBM ThinkPad X24.
Everything seems to be running fine, but the system completely freezes every now and then.
Not linked to a specific action, application or even WindowManager (I've tried Gnome or XFCE).
It was not possible to reproduce a situation to force a lockup - unless, one day I played FreeCiv!
[SOLUTION]
The problem was the graphic adapter:
ATI Radeon Mobility M6 LY [Radeon Mobility 9000]
When changing the display driver from "ati" to "vesa" the system suddenly became stable as a rock!
NOTE: I'm currently trying to run the ATI-driver again, but only using 16bit (instead of the default 32bits)