[Roy Dennis]

It seems to me to be where you are connecting two different types of output from the outputs of the computer. i.e either use 2 vga monitors or two TV's.

As the problem seems to occur when you add the singers screen, could it have anything to do with your aspect ratio settings on the external TV ?
These can usually be selected from the TV settings, nothing to do with your computer settings. Just a suggestion.

Roy.