I hate to be a barer of bad news, but after my years building PCs, I can tell you that 95% of the time a PC crashes after a heavy load is started its because the CPU voltage is too low to handle 100% of usage. I know my rig cant handle a video encode without switching to 3.5GHz from 3.9.
Since your CPU is not unlocked, my recommendation would be to try to downclock the CPU if you can, or try another CPU.
What's the voltage look like while idle? And have to tried opening the Event viewer? Maybe it caught something.