Monitor Probs.

Hello,
My wide screen monitor view keeps flashing from a small size to a larger wider screen size (aspect ratio or something). I have changed monitors and cables in a sequential manner - the smaller monitor just shows a black screen then after a few on and offs of the button on my tower it comes on again, Sometimes I have no picture at all. Is it the graphics card thing and can I run a prog that will put it right or do I have to take it to the shop for a repair?
Merry Xmas
Rog