Re: Nvida TNT2 Pro
As you were moving cables around I would first suspect a loose cable somewhere or even a broken/bent pin on the VGA cable. It might be worth checking all the connections (including making sure that the graphics card is properly inserted into its slot).
If it is one or two specific colours (Red, Green or Blue primarily) that you're having problems with then your monitor might be failing.
Otherwise you might be having issues with the monitor/video card drivers.