From: Trav on 05/17/2011
The first thing to check is if it might be a fan problem. The computer can sense the fan speed and may be shutting it down before it gets too hot, or just because the fan is turning too slow due to dust accumulating on the fan blades.
Easy to check and easy to solve, try that first.
The problem may be the power supply. If it's happening after you added some kick ass video card then that may be your problem. Some people forget that a better video card will need more power in some cases. Sometimes a bigger hard drive causes the same problem. More memory, more PCI cards added, what do you think happens?
Sometimes it's as simple as moving PCI cards around or to other slots, that may make the IRQ order different.
You can try going into BIOS at boot and turning off anything that has to do with power saving modes or suspend / hibernate stuff and see if that does anything.
Look for a Power Save mode on your monitor's settings via it's menu button (on the monitor itself) it may be sending a signal to your computer to go to sleep or something. All these things are now talking to each other, how nice.
Turn off any over clocking and see what that does. (if you had it on)