Many thanks for your reply Brian, you seem to be doing a lot of the work around there. I had been coming to the conclusion that it must be some deficiency in the graphic card or display adaptor that was the problem but the difficulty is to determine what that is. A reasonable graphics card covers all my work without going to great expense and this was one I was given so I am not aware as yet of its full capabilities but has coped well so far. I could try Shotcut with Linux as I have this as a discrete double boot on that computer but its the same graphics card. I could live without it as the amount of cutting I was to do is small. So I may try another card at some time. Thanks again.