Without knowing more about the computer, we can't really say what the issue could be...
A couple of ideas:
- The PC case has poor ventilation and the graphics card is getting really got (it would get noisy so you can probably tell)
- The power supply is struggling and not quite up to the job (usually the PSU will fail or shut off under heavy load)
- Driver issues, which you won't really be able to do anything about
- A ton of background processes which you haven't closed
So, if you aren't sure, a good thing to do would be to install MSI Afterburner and viewing the graphs for GPU usage, and using task manager to see CPU usage.
If the GPU usage is really low and the CPU usage is really high, then the CPU is the bottleneck, and vice versa.
Now, if you say the graphics card is actually freezing and you need to reboot the PC to get it working again, then that would be a PSU issue.
Or, if the game crashes and you have to restart it, then the graphics card may be faulty and you should see if you can get it replaced under warranty (Hopefully this isn't the case!).