Hi, I bought from someone who upgraded his brandnew iMac (late 2015 5k) two RAM sticks of 4GB to upgrade my iMac 27" from 2010. The memory got recognized but then within short time freezes. I realize the memory is faster than the original one (1867 MHZ) but I expected it to simply run on the slower speed but otherwise would work. Anybody else having an experience with fast RAM in an older iMac? Anything I am missing?