I would check Performance level of ram using Memset. Probably find your TRd is 8 for 450 and 500 but at 475 it is 7 (due to different ram divider). Guessing but worth a check. Lower TRd = higher voltage for the north bridge.
The anandtech article is really good at explaining why some configurations require a much higher north bridge voltage. Remember the main component of the north bridge is the memory controller.
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