Did you by chance get a new case with a bundled power supply? Then use that power supply instead of moving the old one over as well?
I've had a custom built gaming computer (my brother ordered the parts online and assembled the computer himself) and it has worked fine for a while. Then, we transferred every part of the computer to another case, and ever since, the computer works completely fine but when running a game (in my case, Starcraft II), the computer randomly shuts down. No freeze, no nothing; it just turns off completely.
Any ideas as to what might be wrong and how to fix it?
Intel Quad Core
GeForce GTS 250 graphics card
Windows 7 Home