We have encountered this issue on ATI Radeon HD3600 series graphics cards. The terrain would sometimes disappear, revealing the sky-sphere, and the terrain tiles would appear blocky if lightmaps were enabled. If this problem applies to you, you should try disabling Vertical Synchronization (VSync).
To do so you must enter the Catalyst Control Center by right-clicking your desktop or the ATI icon in your system tray and selecting Catalyst Control Center. If the application asks which mode you want to use, select Advanced and click Next and confirm by clicking Yes.
In the left menu expand the 3D group and select All Settings. Now, in the right menu, find "Wait for vertical refresh" and move the slider all the way to the left (Always off).
Remember to return this option to default when you finish playing Sacrifice.