Screen goes blank, system is still running but does not respond


Nov 4, 2012

my brand new pc (assembled by a webshop) has random black screens. It seems to happen more while gaming (League of Legends) and even particularly while playing the champion "Katarina" (every five minutes compared to rarely while playing other champs), for those familiar with the game.
The screen just goes blank, all sound is gone and the system doesnt respond to anything but the power button. The LED on my monitor flashes as if the PC is in stand-by.
I've read around on this forum and I think my problem might be something about too much power current running through one cable, so I assume I should send this computer back to the webshop to fix this?

My PC specs are:

Windows 8
Intel Core i5 3570 / 3.4 GHz
Radeon 7870
Monitor is a SyncMaster B2430 Samsung (was purchased separately)

Thanks in advance!