My screen goes blank

Hello,
Thank you in advance for considering my problem. My Vista running HP Pavillion desk to seems to be working fine but after a slide show or some video clips (or at other less identifiable times) my screen goes black. If I turn the monitor off and on several times or toggle through the 'menu' repeatedly it will eventually come back on. The first several times, after I toggle the menu button at least 5 times, the picture comes back on for a few seconds. Each time the 'monitor status' window comes on earlier and earlier until it stays on and the picture remains. I have done a complete recovery since this started and installed some optimization programs. It is happening less but still happening. Again, thank you for your efforts and time.