"No Video Input" crash when playing graphically intensive games

Hi lads, so I have had my PC for probably a year and a half, I got it custom built with a FX 6300, R9 270X and one 8GB stick of ram. The PC has worked perfectly up until a couple of months ago when the screen would start going black and "No Video Input" would show up in a box before the PC would restart itself.
This only happens when playing games like Witcher 3, GTA V, Fallout 4 etc. I've still been playing smaller less intensive games like Risk of Rain, Don't Starve and the like without any problems, also general internet use and all that works fine.
The games run fine, like I could play GTA for 2 hours on 50-60 fps with no issues but suddenly the whole PC will restart iself without warning and it has made AAA games mostly unplayable.
I tried cleaning the dust off the fans in my PC but no joy. What is the most likely cause for this? Do I need to replace my graphics card or power supply? Or could it be something else?
(Sorry if this has been asked before)