Then, I suggest to modify the buffer size.
The file you need to open is in your documents in Warband Folder; It's called rlg_config.
Open it and search for this line:
Render_Buffer_Size
15 is the default setting, this caused problems for some users.
Try to set the Buffer Size first to 50% of your VGA RAM. If the problem persists, increase it to 75%.
Also, you can always try the game in DX7, and make sure you don't have Load textures on demand feature enabled.
You can always try increasing the VGA fan RPM to 80% via a tuner software, I suggest EVGA Precision. Overclocking of the Video Card is not recommended.
If the problem still persists, write down your computer specs, so I can think about it further.