OK. I got it solved! All I need to do was to go to /usr/bin/X11R6/xf86config and run it. I entered all the relevant details - mouse type, keyboard type, monitor type, graphics card type, etc. INCLUDING the horizontal and vertical refresh rates. This seems to have reset all the settings made automatically done by X and the screen now runs fine @ 1024x768. I've got full graphics ability and OpenGL is smooth.

I know I'm new to this, but I found that particaular program easy to configure and it did the job easily. No need to reset X either!

S D