I have an older system, and everything worked fine for years, and suddenly I have a problem with my RAM. I have 2 x 4GB Corsair Vengence DDR3 sticks installed in the correct DIMM slots on a Gigabyte 970A-UD3 motherboard. I am running windows 8 x64. Windows shows 6GB of RAM installed and says 3.98 GB is usable. CPU-Z shows all 8GB. I ran windows memory diagnostic and there were no problems found. I swapped DIMM slots and nothing changed. I tried starting PC with only one stick installed and it starts with one stick, but will not start with the other. This leads me to believe that one stick is bad, however when both are installed it says 6GB installed RAM, which is larger than the 4GB size of one stick(it showed 4GB installed when only one stick was installed). I removed memory from a different PC and installed my RAM into it. In the other PC it says 8GB RAM installed, but still has only 3.98GB usable. I am thoroughly confused. Any help is greatly appreciated.