Since it's a new pc do you have good enough cooling? I would keep an eye on it for a bit with this http://www.cpuid.com/softwares/hwmonitor.html
Your fault for buying an AMD video card.
Have you update your drivers recently?
drivers are pieces of software that make sure your hardware functions correctlyif you just built your pc you will probably need to update the drivers for it
Go to control panel, click on device manager, click on display drivers, right click on AMD Radeon™ HD 7870, click on update driver software, click on "Search automatically for updated driver software" and let it do its stuff
WHaT DOES THAT MEAn
How can I do that.