Probably the biggest things that you haven't mentioned in all that.
How old is this computer? You said you built it yourself but didn't say when.
Which operating system (Windows version) are you using?
To be honest it sounds as if maybe your graphics chip has been overheating and has finaly failed.
I had that happen a few years ago with a laptop the graphics chip failed. As it was an old one still on XP I junked it and bought a new laptop. If yours isn't that old then maybe a repair (under warranty?) may be in order?