[Presto]

It looks like you've eliminated power management as a possible issue. The freezing could be caused by a wayward background app/process. Take a peek at the processes in task manager (CTRL + SHIFT + ESC). See if anything stands out, like a suspiciously named process, or something taking a lot of CPU time when nothing should be happening.

Another thing to try is run MSCONFIG and disable all your startup items. Then, restart, and let the computer sit and see if it freezes. If freezing is inevitable, then this will eliminate this possibility, or point you in the direction of the problem.