When you full screen Toribash - it tries to full screen in the resolution you're playing.
This might cause problems if you're playing in a resolution that your screen can't full-screen to.
e.g. 1024x600 .
So, before you switch to full-screen, make sure you type:
/re 800 600
Or
/re 1024 768
This might also be due to an old graphics card driver.