Computer won't boot up sometimes...

[ojrana]

Hi everyone, I'm having a problem with my computer, at times it does not want to start up. The computer will turn on but no image will appear on the screen. If I turn the computer on and off a couple times then it will start working and boot into Windows. If I leave the computer running for a long period of time, the screen will go blank and I will have to hard-reboot the computer a couple times before it will boot back into windows... Any ideas?

[JJ 3000]

Do you have another monitor that you can try?
Do you have another computer that you can test that monitor on?
When you start it up and there is no image on the screen can you hear the sounds of windows loading?
Also, do the lights on the keyboard flash?
Does the power light in the computer tower come on?