Steam – Borderlands 1 crashing on steam multiplayer

Realize that this game is a bit older, but I'm hoping to play with some friends online!

Whenever I try to host or get invited to a multiplayer game my whole steam client crashes. The game either dumps me to the desktop or freezes and I have to terminate it via task manager. From time to time the game will load into the starting zone and then freeze, telling the other player that I have disconnected.

Single player works great.

What I have tried so far to resolve this:

• Reboot the system after the initial problem
• Verify integrity of game files locally (no problems)
• Update video card drivers
• Reinstall the steam client while preserving the steamapps folders
• Reinstall the game via the steam client (uninstall then install)
• Run steam as administrator
• Run Borderlands directly as administrator
• Run Borderlands EXE in Windows XP service pack 2 compatibility mode (this disabled "Online Game" option – was not helpful)
• Added inbound and outbound blanket allow rules for the borderlands.exe and steam.exe executables in Windows Firewall advanced settings.

Additional troubleshooting info:

• Running Windows 7
• Core i5 4670k / GTX 660ti / 16GB RAM
• A Wireshark packet capture at the time of the crash indicates no unusual traffic between the steam servers and my system.
• Events manager registers no application or system errors when steam crashes
• Steam does not throw any error messages when it crashes, it simply closes in the background
• I have no idea what to make of the dump files; I assume these are the memory states when the application crashed. They correlate with the time of each crash, but I am not an expert on decoding hex.

One last bit – an excerpt from my steam "connection_log" with my steam UID replaced:

[2015-07-08 19:54:08] Log session started
[2015-07-08 19:54:08] [0,0] SetSteamID( [U:1:0] )
[2015-07-08 19:54:08] CCMInterface::OnNetworkDeviceStateChange — Saw device up, connecting to Steam
[2015-07-08 19:54:08] [0,0] SetSteamID( [U:1:11111111] )
[2015-07-08 19:54:09] [0,0] SetSteamID( [U:1:11111111] )
[2015-07-08 19:54:09] [1,2] Connect() starting connection (eNetQOSLevelLow, 208.78.164.13:27017, UDP)
[2015-07-08 19:54:10] [1,2] ConnectionCompleted() (208.78.164.10:27019, UDP)
[2015-07-08 19:54:10] [1,2] RecvMsgClientLogOnResponse() : [U:1:11111111] 'OK'
[2015-07-08 21:40:14] [3,2] LogOff()
[2015-07-08 21:40:14] [2,2] AsyncDisconnect()
[2015-07-08 21:40:14] [2,2] ResetVoiceRecording() (was recording: 0) (last get voice time: 6366 seconds ago)
[2015-07-08 21:40:14] [2,2] Log session ended

[2015-07-08 21:40:18] Log session started
[2015-07-08 21:40:18] [0,0] SetSteamID( [U:1:0] )
[2015-07-08 21:40:18] CCMInterface::OnNetworkDeviceStateChange — Saw device up, connecting to Steam

[2015-07-08 21:40:19] [0,0] SetSteamID( [U:1:11111111] )
[2015-07-08 21:40:20] [0,0] SetSteamID( [U:1:11111111] )
[2015-07-08 21:40:20] [1,2] Connect() starting connection (eNetQOSLevelLow, 208.78.164.14:27017, UDP)
[2015-07-08 21:40:20] [1,2] ConnectionCompleted() (208.78.164.11:27018, UDP)
[2015-07-08 21:40:20] [1,2] RecvMsgClientLogOnResponse() : [U:1:11111111] 'OK'

Any help would be greatly appreciated!