Spudpcker is right. It is an NVidia fault. Last time I just unloaded the Nvidia driver with the 'System/Hardware Drivers' tool and still had the problem. Now I have done a cold restart afterward and lo and behold no problem. I have an old MX400 card running the version 96 NVidia driver. Fortunately I'm not running any 3D intensive software (games) on this old machine but it would be good to know it will be resolved for when I get my next machine.
