Pull the UPS out of the loop and just plug your PC into the wall outlet. If the problem goes away, it's likely your UPS (battery backup) is failing - perhaps the batteries are failing. The UPS electronics could also be at fault, so you need to isolate it for further troubleshooting.
I have never seen this behavior with screen fade and a UPS being used.
As Albert Einstein said, "I could have done so much more with a Big Al's Computer!".