It depends on the PSU also and where it is located. It should be a good one at about 1200 watt to handle 3 x R9 390 correctly. I had a problem with hanging freezing like you do.

I kept the PSU inside a rack and once the PSU would get really warm , well more than warm would cause all start of problems up to screen freezing and nothing happening. Once I opened the rack and give the PSU good ventilation and now the PSU is almost cold all the time, it is running smoothly from a few days.