During 3D Screen Goes Black

My prob is hard to put into one sentence, so this is what my problem is :

Recently ( past 2/3 weeks ) while i'll be playing games on my computer, during the game, the screen would freeze + the current sound that was playing would loop like it does when it freezes, and about 10 or so seconds later, the screen would go black with the monitor error saying "No signal is being recieved" or something along that line, before going into all blackness.

And i know the computer hasn't crashed because i can hear the game playing and responding to me pressing buttons + such.

I dont really know what to do, the fact that the computer freezes up before this happens i'm pretty sure its not the monitors fault + i have opened up the side panel and left it open and already have 2/3 fans in the comp already so i'm pretty sure its not overheating.

The error has occured on both the latest ATI drivers and the Omega drivers

The only thing i can think of is my 3D card, I was just wodnering if anyone else has had this problem and if it could be fixed. ( PS it only happens when i am playing games, so the 3D graphics comes into it, when i'm not playing a game its fine )

And i tried the ATI website for help, TBH its a rip off and the only way i would get a answer there for my prob would be to phone a £1 a minute phoneline.

Any feedback would be appreciated

Thanks, Sheps

( System Specs - Maker = Novatech, Custom Built | ATI Radeon X700 256MB With latest official update | 200 GB Space| 1024 GB Ram |Pntium 4, 3.6 mhz )

+ 17 Inch Flatscreen, not sure of the maker.