Thanks for the replies, atolboo and Anglo.
Just to be clear, there was no "Out of Range" message with the Nouveau driver, just the periodic flashing.
I found another video card, a GeForce2 MX/MX 400 and tried that.
With the Nouveau driver there is still a flashing every 30 seconds, but instead of the screen going completely black some white horizontal lines flash at the bottom of the screen.
With the Nvidia driver there is now no "Out of Range" warning message whilst booting up. The only thing that's not quite right is a small oblong at the top left corner of the screen that does not draw, ie. it is always black.
dmesg returns the same output for both these cards:
[ 471.465731] nouveau 0000:03:00.0: VGA-1: EDID block 0 invalid.
[ 471.465740] [drm] nouveau 0000:03:00.0: DDC responded, but no EDID for VGA-1
[ 471.483319] [drm] nouveau 0000:03:00.0: Load detected on head A
There is no noticable performance difference between these two cards so I'll leave this GeForce2 MX/MX 400 in the machine. This is on my Mums machine because she wants to try Linux and she has a flat screen monitor that will be used when I take the machine back to her, so no big deal about this. I was just curious about why the Nouveau driver wasn't working properly with the video card.