Computer Reboots Itself On The Old Memory

I recently just built this computer so its still all pretty much brand new. Im not really sure what is making it reboot but it seems like im always playing a game when it decides to do it. After somone else suggesting it was the ram i decided to check my ram with memtest86+ and during the scan my computer rebooted itself pointing that it was prolly the ram. So now i am using my old ram while the other 2 gigs is bein sent back for return and the computer rebooted itself on my old memory so now im pretty sure its not the ram. I understand that it could be numerous things such as a loose wire,drivers,crack in motherboard but it would be nice if the problem could be determined or atleast narrowed down.