Since your machine had been running prior to the crash, this is probably not a RAM compatibility issue, but it is still worth confirming that your specific RAM is listed on the motherboard's QVL. Another possibility is that over time, thermal cycling has affected the cpu-socket connections; maybe you had one or more ever-so-slightly bent CPU pins in the socket, and it's causing something unusual like no longer letting the CPU properly identify the RAM you're using. You might want to pull the CPU and use a jeweler's loupe to examine the pins. Any out of place an be nudged back into line with a dressmaker's pin (they've got the big ball on one end, so they're easy to manipulate).