iMac with Mountain Lion freezes

Since I installed the new MacOSX Mountain Lion in my iMac (mid-2007), I'm experiencing a weird behavior.

Usually when I'm doing a heavy usage (many programs opened like firefox, downloads, mysqld, sometimes, not always, a Virtual Machine,...), the screen freezes, the mouse pointer moves, but no action can be done. No mouse or keyboard action take effect since this moment.

But if I connect from another computer by SSH, I can see all the processes running, everything I had opened, is still there (including SSH server I use to connect). From this SSH session I can stop or start everything, but the main iMac screen remains frozen (except the mouse pointer that can be moved). Sometimes the mouse pointer becomes a spinning wheel.

I don't know if I can restart anything from the SSH connection, in order to recover my session, because the only thing I can do, is a "sudo shutdown -h now" from the SSH session, loose all my work, and wait for the switch off...

I've read in other article something about restarting the WindowServer process using a command "

sudo killall -HUP WindowServer

Is this a known problem in the MacOSX? Any coming solution (patch) or, at least, a workaround to recover from the frozen state?