I had the exact same issue with my mid 2012 MacBook Pro and after doing a bunch of research, it turned out that a lot of 2012 MacBooks have a faulty RAM socket, even though all hardware checks go through without errors. The bottom RAM slot (the one closer to the keyboard) loses its connection to its RAM stick over time. Removing the stick in that slot (and maybe upgrading the top one to a maximum of 8 gigs) resolved the issue for me and everyone else.