I just went through the same thing. I to ran MemTest 86+ and it turned up nothing. Turns out it was my GPU causing the crashes due to overheating. Check your temps on all your hardware with SpeedFan or SIW, both are free. Then check with each vendor and see what the heat tolerances are for each of them and compare. I ended up replacing my RAM, CPU, CPU cooler and GPU, trying to figure out what was bad (the hard way). Luckily for me I had extra hardware on hand to switch out, except for the RAM (new).