So I have been having some serious issues with performance of my high end custom gaming pc where everything is slow and i am getting around 2kb/s speed when copying and pasting files internally. Today I bought a second RAM stick (same model) and put it in and now my computer is running slower than it has ever run before. Why? Here are my specs. https://pcpartpicker.com/user/epjkb_games/saved/#view=6YCQZL
As Ralston18 is stating, RAM modules are only guaranteed in the form sold (from the same pack). So it's a 50/50 gamble whether a new set of modules will work with the old. This is because the manufacturing of the modules changes frequently, making them the same make/model/timings is irrelevant. That is just a way to try and minimise potential conflict, but not eliminate it.
I have seen them change the chips from low density to high density and use the same module number. You can create a bootable ISO CD or USB Memtest 86 http://www.memtest86.com/technical.htm and test them overnight. CPU-Z or Speccy will tell you info about what your ram is doing.