I changed the video driver to the one supplied by nVidia and I certainly had no X problems during the few days that it was running.
However when I switched to a plain text console (50x80) I found that each line of text was displayed one and a half times.
Here is a photograph of the screen: http://www.realbritain.f2s.com/xorg/cimg0160.jpg1
I don't know where it gets that text from as it should be displaying a blank screen with a prompt at the top.
So I went back to the Debian 'nv' driver which gives me this effect sometimes:
http://www.realbritain.f2s.com/xorg/cimg0157.jpg http://www.realbritain.f2s.com/xorg/cimg0159.jpg
In the first picture the little green folders down the left-hand side of the top window should be orange, the blue and yellow areas below that window should be all yellow and the xterm bottom right should have a white background and the text is illegible.
In the second picture all the lines in the top window should be white and not alternating with black as shown and the black bars in the bottom window should be green.
I suspect that the graphics effects are a bug in the 'nv' driver so if anyone can suggest a cure for the one and half lines on text in the console using the nVidia driver that may be the simplest cure.