Found this here: http://left4deadforums.com/1873-lag-...p-crashes.html

Recently had a problem with my video card and replaced it. But not before I went through what must have been hundreds of threads about video card issues. I happened across this post which had a lot of solutions to it and I figured I'd share.

First off the most common fix is to make sure all of your drivers are up to date. This includes:

Your GFX driver (for Nvidia stick to a 178.XXX driver the 180.XXX drivers have a glitch in them).
Your Soundcard Driver.
Your Motherboard Drivers.
DirectX 9c or 10 installed

Next if that doesn't fix your problems try which ever seems to fit your problem.

Enable the Console:

Goto Options > KeyBoard > Advanced > Enable Developer Console

Verify Local Content:

In The Steam "My Games" (tab) > r. click on Left 4 Dead > properties > local files (tab) > Click "Verify integrity of game cache..."

Delete Local Content:

In The Steam "My Games" (tab) > r. click on Left 4 Dead > Click "Delete local content..."

Multi-Core Only: Crash Fixes.

Originally Posted by Target Practice View Post
open the console and enter "mat_queue_mode 0" (minus the quotes)

There also is a more extreme version of this Fix.

Originally Posted by Frostedmouse View Post
1) Start the game
2) Alt-Tab to the desktop. <Or press the windows key>
3) Bring up the task manager < Ctrl-Alt-Del>
4) Click the Processes Tab
5) Click Image Name at the top of the list of Processes to Order by name.
6) Find left4dead.exe and right click on it
7) Select 'Set Affinity'
8) Uncheck boxes Leaving only one box checked and click okay.
Congratulations Your game is now running on a single core and shouldn't crash..
Update: You will have to do this every time you run the game.

-Intel Only- Multicore Fix:

Originally Posted by Mutilated Messiah View Post
Check your bios in the cpu settings Deactivate/ Disable "Spread Spectrum" and "Intel Speedstep Tech" these 2 was causing my game to crash to desktop within 15 mins of play.

Vista: Run as Administrator Fix for Random Crashes
Recommended if Both Steam and L4D is Crashing.
Originally Posted by Kertus View Post
The following has solved this problem for some Vista users.
Exit Steam.
Click Start > All Programs > Accessories
Launch the Command Prompt by right clicking on it and choosing "Run As Administrator"
Type in the following command and press enter:
net localgroup Administrators /add Local Service
Close the Command Prompt window and restart your computer.
Then, launch the game again.

Lag:

*Note This can't always be fixed due to location, and ISP
At the main menu hit your console key (Defualt ~) Then in the console type in "openserverbrowser" (without the "s) and hit enter go to the custom tab and re-fresh all and join a server that pings less than 100 to you.

At the main menu hit your console key (Defualt ~) Then in the console type in sv_search_max_ping 100 (default is 250) Should make Launching games from the Lobby a bit less painful.

Connection Issues

Connection with Session has been Lost
Originally Posted by Ktype View Post
There is a file "steam_appid.txt" in the main folder, just delete it, and you are good to go, no more "connection with session has been lost". But every time you start the game it creates another file, so every time you want to play u need to delete that file before starting the game.

Ports Used by Left 4 Dead:

Your network must be configured to allow Steam access to the following ports:
Steam Client

* UDP 27000 to 27020 inclusive
* TCP 27020 to 27050 inclusive

Dedicated or Listen Servers

* TCP 27015 (SRCDS Rcon port)

Graphics Issues: Black Screen or Missing Textures

https://support.steampowered.com/kb_...2537-DLCV-6627

Hanging/Slow Performance for AMD Procssors with Via Chipsets

https://support.steampowered.com/kb_...4268-QWPH-1104

Sound loop: With and With Out Lockups

Try Setting shader detail to medium. (Graphics setting) ~ From DodgeRodge
Try Setting sound quality to medium.
Check to make sure you only have one sound device installed
start > run > dxdiag > If you have more then 1 Sound tab, uninstall the one that isn't your sound card.

Microphone: Microphone works in steam but not in-game (May work in lobby though)

Originally Posted by Nykkie View Post
Open steam, click on Left 4 Dead and hit properties. Then click "launch properties" and put in:

+voice_forcemicrecord 0

Shaderapidx9.dll Fix:

Error: Left4dead.exe has encountered a problem and needs to close.
Details:AppName: left4dead.exe AppVer: 0.0.0.0 ModName: shaderapidx9.dll
ModVer: 0.0.0.0 Offset: 0002df66
YouTube - How to Fix Shaderapidx9.dll Error

[media]http://www.youtube.com/watch?v=rC6RSwdbHFA[/media]

To View a Blue Screen of Death:

1) Start
2) Run
3) Enter sysdm.cpl in the text box
4) advanced
5) Under Start Up and Recovery click Settings
6) Un-Check Automatically Restart then the two OK buttons

After that Your Blue Screen of Death Messages should stay visible until you restart manually.
Pay attention to any DLL files that are listed as that is most likely the causing the Blue Screen.

Check FPS:

bring up the consul and type cl_showfps 1
Check FPS & Detailed Server

Latency(Ping):

Originally Posted by PsyK View Post
Also during game, if you want to see if the lag is yours or the server.

Do
Console > "net_graph 3"

This will display a graph on screen that will be shown like Garry's Mode one.

It has detailed stats on AI usages and other things. Useful to tell what in the map is using the most resources.

List of Known Left 4 Dead Errors
L4D bugs and glitches summary thread - Read first posts before adding - Steam Users' Forums