custom built computer random freezing

[Whitie III]

memtest completed with no errors!

[Whitie III]

Bump

[Geek-9pm]

memtest completed with no errors!

That might mean there is an issue with the hard drive.
Look at this:
http://www.computerhope.com/issues/ch001091.htm
Run some tests on the hard drive.

[Lorraine walsh]

All MBoards have a QVL list for RAM...it means the RAM on their list has been specifically lab tested on that MBoard...and approved.

That sounds like an overheating/processor stability problem. If you overclocked, set it back to normal and do a CMOS Reset.
One thing you could try is Intel Burn Test (http://downloads.guru3d.com/IntelBurnTest-v1.6-download...) which is a stress testing program for CPUs. I use it all the time for testing overclocks, and it just shuts down the computer if it's unstable. If it does this after you've done a CMOS reset, I would RMA the Cpu.