Whether playing the custom map or spawning in as a special infected, L4D2 has the habit of crashing at the worst times. I've done a few things to my own game after getting fed up and I haven't had any crashes or disconnects (unless the servers themselves were reset) in a few weeks now.
For the custom map, the consensus has been that disabling addons is usually enough to get it working.
For overall performance improvements, what I've done on my own system is the following:
- Disabled all addons (except custom maps if you're playing on those nights)
- Disable mutlicore rendering and Anti-aliasing in video settings.
- Add -lv to launch options (Right click L4D2, Properties, Launch Options) Please note: This removes all gore from the game
If that's not enough, here's the link to steam forums post with other steps you can take if you're having other problems: http://forums.steampowered.com/forums/showthread.php?t=1029427
Hope it helps.