iddqdvie, I had the same problem, everybody would leave, I would try to find a new session or join my friend`s session and then the game would crash out of nowhere.
Try this :
1. Disable uPnP
2. Put a static IP address for your PC with this program : Port Forward Network Utilities
3. Put that static Ip address into DMZ in your router settings (Advanced --> DMZ Host)
^^ These things helped me, but I still occasionally get the "connection lost due to an unknown error" issue.
It think it maybe has something to do with the game, because my connection is stable, and if I really lose the connection, then I would be signed off from Social Club, but I don`t. The game just throws me into singleplayer when it happens.
Just to make people aware, disabling uPnP will most likely have a detrimental effect on other devices using the network, and since you're suggesting putting the PC into a DMZ (which is a bad idea, btw) then it's irrelevant. uPnP only affects devices that have to go through the router firewall. In effect, by putting your PC in a DMZ, you are bypassing the firewall so uPnP has literally no effect on it. You should just leave it enabled - it will make no difference whatsoever if you have put your PC in a DMZ.
Your right about the DMZ thing, but if I enable back uPnp, the "everybody left" issue returns ! So I need it to be disabled.