Mainboard: ASUS H87M-E (C2) and ASUS B85M-G (C2)
RAM: Corsair ValueSelect 2 x 4 GB DDR3-1600 CL11
CPU: Intel Core i7-4770S
PSU: FSP Fortron/Source Aurum Gold 400W (supports Haswell CPUs)
When leaving the computer unattended for half an hour or so, the monitor goes to standby. (Suspend/hibernate is not activated on the computer.)
When trying to wake the monitor by moving the mouse or pressing a key, the system hard-locks. The computer's reset button also does not work properly, it takes about 15 seconds for the system to reset (see video at https://dl.dropboxusercontent.com/u/46726302/VID_20130926_192026.mp4).
Before I try to wake the computer up, the existence of IRC logs from the time it was idling shows that everything works as expected. The freeze occurs only when waking it up.
I tried two different mainboards, so it's unlikely that they are defective. The RAM passes hours of memtest testing.
Since the OS IMHO should not be able to cause such a bad deadlock, something else must be the problem.
What could it be? A defective CPU is very unlikely, at best.