Screen blackouts but computer still runs

First of all my basic cpu specs:

OS - Windows Vista Ultimate 64 bit
Q6600 2.4 ghz
EVGA 680i LT MOBO
EVGA GeForce 8800GT (Non Superclocked)

My problem is if I leave my computer on idle for an extended period of time maybe 30 or 45 mins +, my monitor will black out. First however, it will go to my screen saver first and when I move my mouse or hit a key on my keyboard it will come back to my desktop for half a second and my screen will black out. I believe my computer is still on because I remember I left a song on and I could hear the song still playing on my speakers. However, I will have to restart my cpu to get my windows running properly again, but then again another problem arises when I do this. Often times I would say 9 out of 10 times when this happens my computer will load into windows and it will black out again and I will have to restart it twice in a row. I updated my drivers, uninstalled and then re-installed it as well. One thing that I don't have a problem is if I use my computer for a long period of time like surfing the web or playing games. As long as I keep using my computer the monitor will not black out. I turned off all my hibernating and sleeping options on my computer as well, and I just started having this problem about a week and half ago. Please help me out I really don't want to buy a new video card again. Thanks.