Crashes are not usually due to the motherboard and CPU, they tend to be software, RAM and driver issues.
There are many things that can cause a PC to crash including operating systems, software, hardware drivers, etc. The first thing I would do is run Memtest86+ V4.20 or V5.0 overnight to check for RAM issues. If that checks out OK then I'd try stress testing your entire PC.
Many people stress test their PC builds with Prime95, OCCT, or similar to try and find hardware issues. If your system passes a 12-24 hour P95 stress test and still crashes then it's not likely hardware related. Then you need to look at video drivers, software, etc.
With the FX-8120 you also want to be sure that you have a sufficient PSU as these 8-core processors draw a lot of power under full load. You should also check that the CPU temp is not exceeding AMD's specified 61C core temp under lengthy video game play. You should check the operating temp of your video card for the same reason. Any of these issues can cause PC operational problems, typically freezing or slowing, not so much actual BSOD or crashing and re-booting.
Building a reliable PC involves more than just assembling the parts. You need to be able to configure all of the BIOS settings appropriately. This can be quite involved and frustrating as it can require a lot of trial and error with stress testing. It is however often the only means to get a 100% reliable PC.