Screen freezing

[jeffbilling]

I have recently upgraded to Hardy Heron (via CD update + latest updates per internet).
Everything is working fine except for one major problem.
My screen freezes and I have to do a hard re-boot to get started again.
This happens in all sorts of situations e.g. using Office spreadsheets, using Firefox and with absolute certainty when I leave the computer for about 10 minutes and then the screen saver will slow down and then freeze.I don't know where to start looking or what info all you experts out there need to point me in the right direction.
I have an ATI board.
vbe: VESA 3.0 detected.
oem: ATI ATOMBIOS
vendor:
product: o
memory: 16384kb
mode: 640x400x256
mode: 640x480x256
mode: 800x600x256
mode: 1024x768x256
mode: 1280x1024x256
mode: 132x25 (text)
mode: 132x43 (text)
mode: 640x480x32k
mode: 640x480x64k
mode: 640x480x16m
mode: 800x600x32k
mode: 800x600x64k
mode: 800x600x16m
mode: 1024x768x32k
mode: 1024x768x64k
mode: 1024x768x16m
mode: 1280x1024x32k
mode: 1280x1024x64k
mode: 1280x1024x16m
mode: 320x200x32k
mode: 320x200x64k
mode: 320x200x16m
mode: 1600x1200x256
mode: 1600x1200x32k
mode: 1600x1200x64k
edid:
edid: 1 1
id: 443e
eisa: NEC443e
serial: 01010101
manufacture: 16 1999
input: composite sync, sync on green, analog signal.
screensize: 33 24
gamma: 2.200000
dpms: RGB, active off, suspend, standby
timing: 720x400@70 Hz (VGA 640x400, IBM)
timing: 720x400@88 Hz (XGA2)
timing: 640x480@60 Hz (VGA)
timing: 640x480@67 Hz (Mac II, Apple)
timing: 640x480@72 Hz (VESA)
timing: 640x480@75 Hz (VESA)
timing: 800x600@56 Hz (VESA)
timing: 800x600@60 Hz (VESA)
timing: 800x600@72 Hz (VESA)
timing: 800x600@75 Hz (VESA)
timing: 832x624@75 Hz (Mac II)
timing: 1024x768@87 Hz Interlaced (8514A)
timing: 1024x768@60 Hz (VESA)
timing: 1024x768@70 Hz (VESA)
timing: 1024x768@75 Hz (VESA)
timing: 1280x1024@75 (VESA)
ctiming: 640x480@85
ctiming: 800x600@85
ctiming: 1024x768@85
ctiming: 1152x864@75
ctiming: 1280x960@85
ctiming: 1280x1024@85
ctiming: 1600x1200@70
dtiming: 1600x1200@91
monitorrange: 31-92, 55-160
monitorname: NEC E750
monitorserial: 9400198XP

This is output from 'ddcprobe'
:confused: