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.