you might have a boot virus which comes up shortly after boot up and some can be programmed to take away RAM. what you have to do its format your hard drive. if that does work let me know and i will try and find another solution.
Maybe the RAM stick is faulty. Download memtest86, mount the image on a DVD and then boot from it. Remember to test one stick at a time. If the RAM are intact then run a virus scan (you should scan with your anti virus's scanner and Malwarebyte's Anti-Malware), I've never heard of boot viruses though, but rather safe than sorry.
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