SOunds to me like you have the wrong resolution set
or the wrong driver for the graphics server set or the wrong refresh rate for your monitor.
for an easy to understand set of docs on how to set up Xwindows
Try XF86Setup or XF86Config and if you are like me didn't know how to use these then do a MAN on XF86Setup / XF86Config files.
I had many similar problems with a different graphics card
and stepping through the above eventually solved my problem.