Desktop temporarily freezes

The desktop is constantly freezing.

I haven't pinpointed the problem yet. It may happen any moment for no apparent reason. I haven't found a way to reproduce the problem yet.

It is very weird. Some apps freeze before others, but it is different each time. For example the system is frozen as I'm typing this. I can tell because the system monitor applet is not scrolling. The clock and link-monitor-applet is updating, though. Other than this window, no other windows are responding. I *can* switch to other workspaces, but windows don't repaint except this one. Sometimes it looks like a soft lockup.

I read about other bug reports, and, some don't fit because my system does NOT have an ext4 partition and not using an Intel graphics card. I am NOT using Compiz. I am using the open source ATI driver, though.

I opened a different report because I haven't seem any that describes unfreezeng after > 5 minutes. Though it unfreezes, it takes so long that it appears that a hard reset is the only way. So it *may* be a duplicate of another bug.

There are some constants:

1. After 5 to 15 minutes, the system unfreezes. Otherwise, it looks like it's locked (except that mouse moves).
2. While the system is frozen, you can't use CTRL+ALT+Fx switch to the tty.
3. CPU is not busy. It is 99% idle, as usual.
4. RAM looks normal.
5. Keyboard and mouse events don't get lost.
6. Network looks normal. I can SSH to the host. I put a laptop in between to monitor the network traffice (as maybe a DNS request was blocking some critical system component) but everything came out normal.
7. Nothing goes in the logs during the freeze (I put some tails -f to logs from Xorg, messages, debug and a watch to dmesg and it didn't show anything). Sometimes a "Resetting GPU" comes up in the logs, but I think this is a consequence (of the system being unresponsive) rather than the cause.

I made the following tests in order to try to isolate the problem, but the system didn't freeze:

1. find /
2. 2 x burnP6
3. 3 x burnP6 + 2 x find /
4. swapoff -a; swapon -a, asuming (swap_usage < free RAM + disk_buffers).
5. Loade 15 gnome-terminals with full scrollback buffers (~60 MB each) and making all of them read a big log file at the same time.

However, I found that CPU never went past 25% in a quad-core. I assumed all of them used the same core, so I loaded a couple of xterms too besides the gnome-terminals and still it didn't go over 25%.