It could be the monitor but you could also consider the lead, graphics drivers and graphics card(s).
You could also try a different monitor as this may confirm the monitor itself.
If the problem is still present or a monitor is not available, I suggest you try a different lead first and see if this clears it. If this fails try uninstalling/reinstalling the driver for your graphics card (you could do this by rolling back the driver click here - for XP audio but similar for Vista/W7 graphics - and then reinstalling). If this fails try removing reseating the graphics card(s) click here.