DSUB Power saving mode only when gaming

[Tombstone265]

Hello, I'm new to the site mainly because i cant find a fix to this problem. I've searched and i cant find anything. =/

Well, heres my problem. My computer runs fine when im just browsing the internet, or doing simple things i guess you can say. But whenever i run games, for example, Battlefield 2, secondlife, etc. It'll run within the range of 5-30 mins then my screen will go black, then switch to "DSUB Power Saving Mode". The only way i can exit out of this is by restarting my pc. I know my computer is able to run these games because its worked fine before, but it randomly started doing this. I've also tried several monitors so i KNOW it has nothing to do with the monitor.

My computer isnt too great, but i dont know what could be causing this, all my fans are working, comp doesnt seem to be overheating, If anyone knows anything about this please help, i need some advice on what to do or what this could be. No other forums have answered me, =/

I'm running windows xp, nvidia geforce 6600 gt, my power supply is a 600 watt, thats basically all i know without looking into my computer.

I'm planning on buying a new motherboard and video card soon, but id like to know whats going on here.

Thank you.