It sounds like mismatched RAM or bad RAM sticks.
Have you run Memtest86+ on the RAM? You can run a test on each stick in the first slot to test each stick individually, then run a test with more slots populated.
I was going to jump right to a defective motherboard, but now you say you ran 8GB previously, so I'm leaning more towards mismatched RAM.
FYI: if the system freezes during BOOT before the hard drives are found then the OS has not been loaded and cannot be a cause of the problem.