Could be a lot of things really.
Had you had any error messages produced in the installer log when been installing the game? Had you tried reinstaling it keeping your saved folder intact? Be sure to use 7zip File Manager from 7-zip official package to check the integrity of the BlackMesa.7z before proceeding with reinstall. Before trying to reinstall you could try to change your gfx settings and check if that helps.
Set HDR to None, turn off motion blur, turn off bloom, set shadows details to medium or low, set models to medium or low, e.t.c.
And I assume you have the latest version of the GPU and sound card drivers installed on your system, do you? Make sure your GPU driver is at least 301.42, there were reports by other GeForce mobile GPU users concerning BM crashes due to outdated nVIDIA drivers.