GPU problems - Software or Hardware automatic restart

So I have a problem with my GPU it is a Gigabyte nvidia GeForce gtx 780 basically I only just bought it at Christmas and ever since a problem has occurred. After I installed the drivers for my GPU and after a quick restart when I try start windows again it restarts at the login screen sometimes I can get a bit further but this only occurs when I download the drivers and plug it into the graphics card I've looked around a bit and people have said many things like it was the PSU or something else I personally think it is a software problem but I'm not sure what though I've been trying for a week to fix it now and before I use to be able to get into safe mode while plugged into the graphics card now it crashes while it's loading I can use the the integrated graphics from the motherboard while the drivers are installed but not the GPU I've also checked this in a second computer and again it restarted at the login screen somone has told me to send it back to gigabyte but I bought it off eBay brand new from a guy who sells many other products with 100% positive feedback and I certain it's not the GPU that the problem is occurring at but I'm still un sure any help would be appreciated.

My specs:
Motherboard - Supermicro x8sti
CPU - i7 920
GPU - nvidia GeForce gtx 780
RAM - Gskill 2x4gb
PSU - corsair cx750m 750wats
Hard drive - western digital 1TB

Thank you in advance..

I have deleted them all in control panel