Recently my monitor screen has started to flicker, with the desktop picture contracting from top and bottom while it does so. It'll do it for a bit and then be okay for a while, but seems to be happening more frequently as time goes by. I haven't changed anything in the setup and I haven't placed any speakers, etc. nearer to the screen. Does this sound like the graphics card might be on its way out, or the monitor?
Sounds like either the frame sync is going or the frame oscillator has changed so that the syncronisation circuitry is having trouble keeping it on frequency. I suggest that you try your monitor on another computer to see if you get the same result.
Hardware or Monitor change it could be the Display properties that have changed, or the refesh rate within the display properties.
I had similar problem when I got a new Iiyama 17" monitor and they told me the AGP card was not up to the job. I am currently waiting for a new AGP card.
I would if you can try someone elses Monitor 1st. Then if no improvement replace the older item either the monitor or the AGP/PCI card. If its onbaord AGP VGA maybe the shared memory level is strained.