BIOS only recognizing half of my RAM

[inveigh14]

I bought and assembled my computer earlier this month and everything has been working fine until this morning. My computer would restart after about 20 minutes of being powered on, but with no blue screens. I went into BIOS (I have just had my settings set at Optimized Defaults) and noticed only 8gb of the 16gb I have installed are being recognized. I tested each stick in slot A1 and all were recognized as 4gb. I tested each DIMM slot as well and they were all recognizing 4gb. But when all 4 are occupied it only recognizes 8gb. I tried removing just one stick from each slot at a time (leaving 3 occupied) and still only 8gb are being recognized. The memory frequency is also reading as 0 mhz. When it should be 1600 mhz. I have taken out 2 sticks (leaving slots A1 and A2 occupied) of ram and have had no problems since (as far as random restarts), but I'm unsure of what is causing this issue and would like to have all 4 sticks installed and functioning properly. Thank you, in advance, for any and all assistance. This is my first build and all help is most appreciated.

CPU: Intel Core i5 3570k
Motherboard: Asus Sabertooth Z77
Bios: 1616
RAM: 16gb Corsair Vengence 1600mz 4x4gb
GPU: EVGA GTX 670 FTW
SSD: Crucial M4 64gb
HDD 1: Western Digital Caviar Black 1.5 tb
HDD 2: Seagate Barricuda 1.5 tb
PSU: Corsair HX750

 

[rolli59]

My suspicion would go against the motherboard first.

 

[C12Friedman]

^+1 Going to agree here
As an option, If you haven't yet, run all four sticks (two at a time) through memtest available here
http://www.memtest.org/
You'll probably want to do it for 8 hours each time - while you're sleeping or at work is good since your computer will be dogging during the tests - could be something a little easier

 

[inveigh14]

I'm still getting restarts actually. It seems to be every 20-30 minutes, otherwise I would definitely run memtest. My best guess is the motherboard too. I'll give ASUS a call in the morning and get this thing RMA'd I suppose. Thank you very much for the quick responses!