First - RAM.
Have you checked that XMP is setting up the RAM right.
According to website - should be 7-8-7-24-2N
Maybe something getting set wrong.
However - I'd agree with DM - your PSU isn't the best
Wattage isn't always the best way to look at your Power supply - but gives an indicator..
Key here is what are the individual voltage rails capable of delivering.
Found it hard to track down your PSU rating specs - but eventually found this
http://www.overclockersclub.com/reviews/corsair_hx520w/3.htmFirst off it has 3 +12V Rails. You need +12V for your Grfx, fans, Harddisks, and optical drives. (memory and others via the ATX-12V connector)
Each of the 12V rails are for 18A. Your Grfx card takes 2 - so 36A from 2 of the ports.
However look at the manual for your Grfx card I see this "
Minimum 600 Watt or greater system power supply (with a minimum 12V current rating of 42A) is recommended."
This suggests to me that your PSU is borderline as the GPU is going to steal that additional 6A from somewhere.
Would recommend looking for something with 22A per 12V Rail if you go looking.
(You wouldn't have seen this with an ATI 6870 or 5870 as they are ok with 18A)
My suspicion is your drawing too much to the GPU periodically which is pulling from the 12V rail to the 8pin ATX12V connector on the board and thus pulling power away from components like your memory.
In short - get a new PSU.