I know that this thread pretty well solved itself, but I was having some pretty crazy problems with my server computer for the longest time, would only last a day or so without a reboot (it runs Linux, and typically had uptimes of over three months). So I started replacing all sorts of hardware, and it would still crash on me. Even replaced the powersupply. Until I realized, that I hadn't checked the thing that is easy to check and kills systems all of the time, ram.
In order to test your ram in your system, use a program called memtest, it comes as a bootable cd image, so just burn it and keep it around, let it run overnight on any system you are having problems with, and if it doesn't find any memory problems, you are almost guaranteed to be safe. Being a totally free program also helps to use it first before replacing a lot of stuff:
http://www.memtest86.com