As everything seems to be OK when you run your RAM at 1333 Mhz I would suspect that the memory controller is faulty and that is built into the CPU. The Phenom ii CPU's are officially, only rated to run RAM at 1333 Mhz and anything above that is seen by AMD as technically overclocking. If you haven't read the sticky at the beginning of this form here:
http://forum.giga-byte.co.uk/index.php/topic,2515.0.html then it might be worth checking it through.
Sadly, the only way of knowing if your CPU is faulty is to test a different one or to return it to AMD for testing which sounds fine in theory but becomes really difficult in practice as you either have to access to another processor or be without your PC for a couple of weeks while the processor is tested.
Of course, there maybe nothing wrong with the CPU and it would be a good idea to contact Gigabyte Technical Support to see if there is a known BIOS issue or another reason why this is happening.
