Screen going blank and system freezing

With increasing frequency, something is happening when I am playing games or watching movies. The screen suddenly goes blank and displays the message that it is not connected to any source, but the audio continues for a few seconds before it freezes in a quick loop and the only thing I can do is restart the computer. Obviously this is incredibly disruptive to a gameplay/movie experience -- what is happening, is my video card dying?

Windows XP SP3

AMD Radeon HD 6850 - 1024MB

MotherBoard:
ASUSTeK Computer INC. P5K SE Rev 1.xx
Bus Clock: 200 megahertz
BIOS: American Megatrends Inc. 0401 07/19/2007
Realtek Onboard Audio

3328 Megabytes DDR2 RAM

Processor:
2.00 gigahertz Intel Pentium Dual
64 kilobyte primary memory cache
1024 kilobyte secondary memory cache

You might want to try another power supply...