computer stops working, and screen goes black. Any suggestions?

Thanks in advance for any help! I'll try to explain as best I can. I have a custom built pc, running Windows 7 ultimate 64 bit, 8 GB RAM, Intel LGA 775 motherboard, Intel Xeon 2.4 Quad processor, 550 watt power supply and nvidia GeForce 9800 GT graphics card. I do audio recording, and use an external audio interface (novation x-station 49). My computer has ran flawlessly for the past year or so, now my 2 (22" Samsung) monitors go black after a few minutes. It started out once or twice a day, now it only takes about 5 minutes.

My first thought was the video card (because both monitors go black) which I'm still in the 5 year warrantee period, so I can get it replaced. Everything is normal, then I get a second of messed up screens, then it goes black. When the screens are messed up, they look like the one referring to bad video card ram in this link... Diagnose video card problems by comparing with example corrupted screens

But!...
When it goes black, I have no audio either. ?

I cleaned the dust out, ran with cover off, all fans are still running, and computer appears as its operating as normal, except the screens go black and audio stops.

...so I guess my question is, if my video cards ram is bad, and it crashes/overheats and stops working, would it cause the audio to stop also? Or could it be something else, like the mobo, or cpu?

I also don't get a error message, and don't see it in error logs, but have to do a hard re-start to get it back on and working for a few minutes at least. :'-(

Thank you for welcoming me here! And thank you for your effort to help me!
Actually, I have a metal rack mount case. I have not installed anything new (except Windows updates), and have no games installed (dont have time for games unfortunately). I haven't done any hardware changes for over a year, and I haven't moved the computer for months.
The problem was consistent, and a regular occurrence. After 2-5 minutes, I'd get a checkerboard like distortion of the screen for a split second, then it goes black on both monitors. After the screens go black, the computer appears to be running. The lights are working showing hard drive is active, and fans are all still operating as normal. It didn't matter if I was running programs, surfing the web or sitting idle.
I sent my video card in for replacement today. Hope that fixes it, if not, I'll be back. I just hope it's not the mother board and/or processor.

I had the exact same problem as you and it ended up to be the video card (again same one). Card looked fine and clean but when i finally did an autopsy and removed all the pretty cover it was blocked solid with dust.
Art.

It was the video card that was bad. Must have over heated, and been a problem with the cards design, as PNY nVida replaced it (free of charge) with a different card, and now problem solved. Thank you all for your help and advise! Cheers!