Update with more information: While people previously didn't experience the issue while playing with the user, they began experiencing it while playing with me AND that user. However, the issues did not seem to happen if the third-party was in a party with only the afflicted user OR I.
I know...
no idea why, but when I'm partied up with a particular steam user (https://steamcommunity.com/id/SomeDumbSkrub/) my game seems to disconnect every 2-3 minutes. If we manage to get through the queue into a game, the issue clears up after I'm forced out of the party from the game crashing and then...