Brand new laptop randomly shutting down and a square appears after

Hi,

So I purchased a new ASUS laptop just last week. My control panel says it is model X555UA. It came with 4gb of RAM, but I upgraded to 8GB of ram in the store when I bought it. It has been great, except the last few days while I am using it, the screen will go black and the laptop will just stay black for a few minutes, then my log in page appears. Of course, when I log in everything I was working on was no longer there as the laptop must have restart itself. As well as this, when the laptop turns back on, a coourful graphic type square appears for a few minutes..never stays longer than about 20 minutes.

Does anyone know what the issue could be? I know I should take the laptop back, and I probably will, just wanted some further info before I did as I really can't afford to not have a laptop for a week or so while it gets fixed. Any information is appreciated! I am not very computer literate so sorry if some thins don't make sense. Thanks.

Update: After I wrote this, the square popped up again! So it has reappeared....and literally just disappeared again as I typed this! Could this be a faulty graphics card?