http://www.pdsys.org/blog/post/2009/02/07/Fallout-3-VideoDisplay-Freezes-sound-keeps-going.aspx


Open up the fallout.ini file in: C.\User\My Documents\My Games\Fallout3
Find the line:
bUseThreadedAI=0
change it to:
bUseThreadedAI=1
Add another line after it and insert:
iNumHWThreads=2
This will limit the game to 2 cores and prevent the engine bug from causing the game to freeze.