Monitor Blurred and doesn’t fix without restarting PC

Hi,
I have a Samsung Sync Master 957 MB CRT monitor. I am using it more than 5 years. Now I am facing some problem with it. When I first start the monitor it works perfectly, but after a few minutes it goes blurred and I am unable to see anything clearly.
I have to turn it off then it’s ok but after some time this problem occurs again.
What do I have to do?
