Do you have an Asrock motherboard? I ran into the exact same problem as you, where my computer would constantly reboot without warning every couple of minutes & I also would only see 3.96GB of ram usable out of 8gb. I was frustrated for days trying to fix this issue, tried everything from reinstalling windows 7, reformatting to windows 8, switching the ram around to searching days on the internet for solutions.
Turns out all I had to do was turn on X-Boost, a function you could enable when you first boot up you're computer (keep pressing X when you turn on your computer). This fixed my problem completely, haven't had my computer shut down ever since & it also fixed my ram problem, the full 8gb is now readable.
I'm 100% sure this is the solution because I had built two identical computers for me and my brother.
CPU - AMD FX 6300
GRAPHICS CARD - Gigabyte Nvidia GTX 660 OC
MOTHERBOARD - Asrock 970 Extreme4
RAM - G.Skill Ripjaws X 1600Mhz 4GBx2 (8GB)
PSU - Antec Neo Eco 520c 520W 80Plus Bronze
Old bump but hopefully this helps out other people before they assume their parts are faulty. If you don't have an Asrock motherboard and run into this problem, maybe your motherboard has a similar function to X-Boost at the boot up screen?