You probably know more than me since I have never built a PC, although I intend building a server for storing my work online.
It is very annoying when things don't work and the tendency is to panic, because you know it will be costly/take some time to straighten things out again.
However the trick is to be methodical and eliminate possible causes to the obstruction one step at a time, and be conclusive in your diagnosis. Also close observation of the symptoms of any failure is necessary.
One other thing I thought of. You said it was not overheating- but anyway I suggest you install
Core Temp and see if you can monitor your CPU temp while you are playing various games.