Memory Problem on 790fx-gd70 - how to access DDR Voltage in BIOS?

Question

I have been getting memory related problems on a 6 month old system. I could only boot from one stick of RAM (Corsair TW3X4G1333C9A) but even then system was not stable for gameplay or stability test though this stick did pass overnight memtest. I returned the board, cpu and memory for them to test. They diagnosed faulty RAM and replaced. Well it worked OK for 24 hours then I started getting BSOD "PFN_List_Corrupt" and "Memory_Management" crashes.
I tested the new sticks and one fails memtest after a few seconds with 2378 errors and the other, although it passes memtest with 20 passes, crashes out straight away on Prime95 Blend Test (i.e. the one that stresses RAM).
They suggested I enter DDR Voltage manually rather than relying on Auto but no matter what I seem to enable this option is greyed out. Does anyone know which settings need changing from defaults so I can get at the voltage section?
I have tried swapping around the dimms but they are all the same regardless so not a slot problem. Now of course, I don't see it as a RAM problem either unless I am the unluckiest person to keep getting bad sticks. So now I think maybe something on the board or CPU is causing damage to the RAM or at least causing it to mis-report. Could a fault in the memory controller be doing this and if so how can I test for this?
At my wit's end here. Hope someone has some ideas - either on how to access DDR Voltage in the BIOS or get to the cause of my problem.
Many thanks.

Answer 1

OK thanks for explaining a bit more  .  This power unit did get some very good reviews in the sub $100 from many sites and from these I thought it would suit my needs. I have arranged to try a friend's 650w next week.
I think I am maybe not being clear.  I have had the system running OK for six months. I had had no need to test the ram and didn't bother with stability tests at that stage.  I am not a gamer and use my machine more for video and graphics work.  One thing I did notice from the start was the couple of games I do have on (Call of Duty and Fallout3) would give occasional freezes.  I put it down to my now rather old hot GPU which I would get around to replacing in due course.
Now the real problems started recently when I got round to replacing the stock cooler with a Titan Fenrir.  Trouble with this is it overhangs Dimm1 but in moving around the memory sticks I seem to have opened a real can of worms.  I could only get it to boot from one of the sticks.  This stick passed memtest but would crash out of prime95 and games would freeze within a couple of minutes. 
Now, the retailers here in the UK told me to send back board, cpu and RAM.  They tested all 3 but they only replaced the memory and as you can see from my first post the memory related problems have continued.  The memory by the way is in Corsair's list as "100% tested compatibility".  Is it possible that something about the CPU/Board that is damaging the RAM?  If a stick is failing memtest (it failed another test too called Windiag) is that definitive?  It is bad for sure or board/cpu misreporting?  Am I right in thinking memtest86 does cannot distinguish faulty RAM from faulty memory controller?  Is there a test I can do to check if the CPU alone is faulty?
I don't know if this means anything to you guys but when I ran the prime95 (Blend) recently with both sticks in it failed straight away on 2 cores but went on running on the other 2. 
One of my other frustrations is I don't know anyone else with am3 board or memory so swap things around.
Any suggestions for pinning this down would be much appreciated.  Just don't assume my knowledge is as great as yours  .  But I do learn.  I never knew v x a=w for example. And I now know how to change DDR Voltage in the BIOS - I had been expecting a sub-menu where Auto was greyed out and in fact it was just sitting there waiting for me to press + and -.  Very embarassing that I spent hours trying to change this when the instructions on what to do were right there in the help column!