[cucubelu]

What volume do I need to set?

mat_dxlevel changes the DirectX mode used within a Source game. The player is only able to use modes that his/her hardware supports. The format is
mat_dxlevel [DirectX version]
mat_dxlevel 70
mat_dxlevel 80
mat_dxlevel 81
...

For DirectX 9 the value depends on the hardware's capability:
mat_dxlevel 90 - DX9 with Shader Model 2
mat_dxlevel 95 - DX9 with Shader Model 3
mat_dxlevel 98 - DX9 on DX10 hardware (SM 4)

This can also be specified with Launch Options, use +mat_dxlevel [DirectX version] on the launch options of any Source game to do so. Alternatively, you can use -dxlevel [DirectX version], however this specifies a number of extra presets, including resolution and quality.

Try some of these values, all I know for sure is that people with more modern system find 9 a lot more stable, but I have even read that some people managed to set it to dx11 to stabilize it for their system. However, don't know the values for those.

Also set it in launch options -dxlevel 95/98 (etc)

Hope one of these work for you.