If I'm understanding this setup, the Macbook is connected to the PC only from its line out to your PC's line in, with nothing else. I'm also assuming this is a desktop PC with a discrete graphics card. And if it isn't, it'll be helpful to know the exact setup
I would try replacing the audio cable first, preferably one with chokes on both ends. Or trying this with another device in the line in.
I can't put the blame on the DVI cable, you can read my explanation in the spoiler box.
DVI uses a continuous data stream in a format similar to VGA, i.e., some syncing and RGB values. Even if you move the mouse or whatever, the signal wouldn't change much. Plus DVI operates at 1.2 Vpp (A picture of a DVI signal
), which is too low of a voltage to cause significant amounts of EMI.