Sorry, I have no idea why your computer only sees either the 2 4GB sticks or the one 8GB stick for a total of 8GB.
I'm also not up on what tests you'd run to stress the hardware and discover what is causing the freezing. You'll have to wait for someone else to post that information.
Try running MemTest86+ and see if it reads the RAM as bad. Will need to rung it for several passes. It may pass the first few then fail on the next.
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