Crashes Only In Multiplayer

Hello, a friend of mine and I both seem to be having odd issues when playing with each other. We can play for a few minutes, sometimes for hours, but without fail eventually the game will crash for one of us. Single player works just fine for both of us, but when attempting to co-op with each other we’ll get an odd disconnect that crashes the game entirely. Note that only the person joining will crash, whichever one of us is hosting will never experience this sort of crash.

More details on what we’ve been experiencing and what we’ve tried are here:

And my system specs are thus:

GPU: Asus GTX 980 Strix

CPU: Intel i7 4790k

Motherboard: Asus Z97