Monitor Problem

System Configuration
Intel D875PBZ
3.4E CPU
2GB Memory
nVidia 6800GS
Samsung 19″ LCD owned less than six months
My problem is after startup and system is running my screen will go blank for a second as though it was refreshing,this will happen two or three minutes apart,after that system will hang and go blank,the system is still on,but their is no video.
What could be causing this prolem
Help
Thanks In Advance