If it's not your video card, look around on your board, at the little capacitors (the little cans) on the board and see if they're uniform or if one of them is blown or has expanded. Some of them have a safety 'x' marked on the top of them to help contain a pop or expansion when it goes. Not a good sign. But then, you know the fault's on the board. If the board looks good, it could be a power problem. Next is to check your power supply unit and power train.
By the way, tell us the specs on your psu. A bad psu could be the problem, too. I had several power supply units go bad which resulted in black screen crashes.
Your HIS Radeon R9 270x uses about 180 to 250w. AMD recommends 500w which is a bit much but on the safe side (probably AMD is thinking about crossfire options). I'd say anything 350-400w on a single rail is safe, considering board support/cpu and peripherals added.
Is the electricity in your area stable and reasonably clean? Are you using a power strip or UPS unit? Does the wall lights flicker in the room? Sometimes the power or power devices fail or go out slowly and goes unnoticed. For a short test, plug it into a known good power strip and run the game a couple of hours. If the electrical is good, board and gpu is good, take your power supply unit out and put it in another computer and see if you get the same crashing results. Or, use a spare psu or borrow, or take out a psu from another computer and run it your game computer.