See screenshot: http://www.more-solutions.co.uk/files/12032010325.JPG
I have tried two identical PCs in case the problem is system or video memory.
Initially I had the monitor through a KVM, which gave me a lower resolution but usable screen, although with square blocks on the screen where it wasn't updating properly. (I tried to open a terminal window and just got a white box which I could move around but could see nothing in the box.) It was when I bypassed the KVM and rebooted that it identified the screen and gave me a decent resolution login screen followed by the linked screenshot after login.
Chipset is Intel 82810E DC-133 (CGC) according to /var/log/messages
Suggestions? I'm hoping the screenshot will tell someone exactly what is wrong, but I can't google a screenshot and can't work out how to describe it in words!