Try running OCCT CPU, GPU, & PSU stress tests while keeping an eye on PSU Outputs, CPU & GPU Temps, Fan speeds, and for anything abnormal.
Furmark puts a lot more stress than other stress testing software in its stress tests of GPUs which may cause it to overheat during the test. This I have read in several websites about Furmark. But possibly Furmark has updated the software to avoid this type of action.
OCCT PSU Stress Test will indicate if you have a power or temperature issue by the PSU Outputs during the stress tests and whether your PC crashes or the test fails.
The PSU Stress test runs both the CPU & GPU Stress tests at the same time putting maximum stress on the PSU.
The problem is caused by the Enermax Revolution DF 850 power supply not being able to handle the high transient loads by the GPU when running Furmark. You will need to upgrade the power supply or not use programs like Furmark which pull way more wattage than most normal applications.