There's a very common error known as "No Free Edicts," that may crash the game if you're at the end of The Mall (Stage 2), or starting at Clean Up (Stage 3).

There's no fix I know that would solve this except for uninstalling mods that can conflict with files that are of the same kind, or completely playing without mods (vanilla mode).

If you want to skip the third stage and onwards to the fourth, Undead Center, follow these instructions as this will help you in single player and multiplayer.

1) Via console:
changelevel l4d_dbd2dc_undead_center

2) Via Admin System:
!console changelevel l4d_dbd2dc_undead_center
OR
/console changelevel l4d_dbd2dc_undead_center

Within Admin System, typing a command with the slash in front prevents your entire server from seeing your command. With the exclamation mark in front, your server will see it.

I'm sorry again, I'm not sure of another way to entirely play Stage 3 without unbloating the game.