Your mixed RAM is a big red flag to me. Looking at the RAM info, you have two Samsung modules, and two Kingston modules. They're probably both totally fine on their own, and MemTest likely won't find any issues, but generally you need to be very careful about pairing RAM. Even though they're both the same speed, I'd still say they're likely to cause you grief. Try removing the two sticks you added, I assume they're the Kingston modules, and run your system on 16GB for now, then let us know if the crashing persists. I know you said it was working originally with the two of them in there, but I'd prefer to eliminate that as a possibility.
