Your saying you ran the mem test?
How many times did you run it?
It's better to know if the RAM is going bad then to keep searching for other things, it will take 4-5 test runs to get a good test.
Read this thread, there are a couple of things to check. Crashing daily (The computer has rebooted from a bugcheck) Windows 7 blue screens and other woes
Try re-loading all your mother board drivers
, chipset, check if there is a new BIOS version.
Let us know if any of these helps.