@Tarlach: To confirm, you are saying that the system crashes when there are a lot of resources in use? Are you getting a BSOD? If so, what does the BSOD say? If it isn't a BSOD, is it just the application crashing, or the system is just randomly rebooting?
What happens is he plays for a while and will sometimes get low resources warnings from windows (a little popup from the sys tray) despite only having the game and steam running (not even a browser). If he keeps playing or if he starts the game again to keep playing (sometimes he gets a crash to desktop) he'll get a BSOD referencing igdkmd64.sys. Previous to Windows 8.1 he never had a problem in (too many) hours of gameplay and nothing changed other than moving to Windows 8.1. He's gotten this playing Terraria and Team Fortress 2 (that's mostly all he plays). I had the same thing occur on my PC when my nVidia 660 Ti was removed for repair though with my 600 Ti back my primary problem is a black screen when setting the primary video card in the bios to PCIe (see the black screen thread).
Like I said in the other thread, I don't really want to send my PCs, but I'm happy to upload a minidump or any other kind of debugging step I'm able to do.