From your description, it sounds like the problem stems from the video card. Video cards are also designed to process audio. And while I am sure you have USB audio selected because you use a DAC, that does not mean you won't necessarily get interference from the video card. What video card is it exactly?
You also mentioned that the video card is taxing your computer during games, as in drawing a lot of current. I'd be curious to know exactly how much current your PC is pulling. Do you believe you have a large enough capacity power supply to handle the load in the computer? Do you have a quality video card and motherboard in your computer?
Sometimes cheap motherboards and video cards can cause more problems then their worth. But if I had to guess, I'd say that you probably have a power supply that is being taxed by the video cards draw, and its pushing the PSU to the limit, which is not good, cause that will cause premature failure of the PSU. Hopefully you can get this figured out.