My monitor goes of,but cpu is running...

Hello,
I am vignesh,let me describe my pc

Processor:Pentium(R)Dual-Core CPU E5300 @ 2.60GHz
Harddisc:360 GB
RAM:2 GB

I have a philips monitor lcd 191E.
let me come to the problem...
When i switch on my PC it starts well.When I view a photo in full screen or play a video or play a game(eg.Chess provided with os windows 7) my monitor just goes black and shows "no input signal","monitor going to sleep mode".my cpu seems to be running well, all the fans are good, sut i have to restart my computer and it comes back , same problem occurs...
i have changed monitors,and tested my ram everthing seems to be good but the problem still occurs.Please someone help with this problem...