My screen goes Black when watching videos

As the title says, my monitor screen goes black, the video stops playing (audio turns off), and I can't do anything. The PC is still working, but there's no signal on the monitor (the power indicator goes yellow). And I can't do anything but hard reset the PC, to get it to work again. Now this ONLY happens when watching videos, I've tried gaming for hours, reading texts, and the PC works fine, but as soon as I start watching a video, after 15 - 30 minutes the problem occurs.

Now, I always keep track of temperatures, and my CPU temperature never goes over 60C and my GPU always stays under 65C (when gaming or watching videos). I have also tried running FurMark a couple of times in a row, and the screen never goes black, it ONLY happens when I'm watching a video.

My Specs:
Inter Core 2 Duo E7500 2.93 ghz
MS-7329 Motherboard
ATI Radeon 4850 HD 1Gb DDR5
4 GB of DDR2 800mgz Ram memory
MSI Power Supply Unit 550W
Dell 1908FP Monitor

Does any have any idea what could cause the problem I'm having, and any idea on how to fix it? If you need more info, please ask, THANK you!