Things to check:
1.) Maximum Memory is unchecked. (msconfig ----> boot ----> advanced options)
2.) Memory Remapping enabled in BIOS.
Things to try:
1.) Clear CMOS
2.) Update BIOS
3.) Reseat CPU (it has helped solve this type of problem before)
4.) Make SURE that you are running 64-bit (Sorry, have to say it)
Also, did you run memtest? If not you might want to run it for a few hours. Probably not the culprit here, but if nothing else is working then you might want to.