Download and run a memory test next. Since before the motherboard replacement you were getting errors that seemed like memory related, I'd rule that out next. Let the test run over night in a contant loop and it will record the errors.
http://oca.microsoft.com/en/windiag.asp
and Memtest has a few versions out there too.
http://pcsupport.about.com/od/toolso...memorytest.htm
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