CTD W/ Medium Vegetation

[Isaac Cisneros]

OS: XP SP3
GPU: Radeon 9200 128MB (I know, it's old! )
CPU: Intel Celeron 2.4Ghz
RAM: 512MB

My settings are High Terrain, Medium Vegetation, Unit shaders ON, Everything else is to the lowest setting.
Sometimes I even turn down de RES to 800x600 .

I just downloaded RSII, and updated to 2.1a and then to 2.5b.
The problem is, that when I put Vegetation Quality to medium, it CRASHES.
But it doesn't happen all the time, sometimes I change Veg quality and nothing happens.
The RSII launcher VEG quality is on the lowest setting.

Is there anyway it wont crash every time I use MEDIUM VEG quality?
I know my computer it's old, but when it doesn't crash on MEDIUM VEG quality, it runs the game well.

[Paedric]

It only happens with Medium, and not with High?

Also, when do those crashes happen, during battles?