Yep, and I know the answer. I am playing it right now on my rig and went through the exact same thing. The problem is apparently with a patch they did, which has trouble with quad core CPU's.
To fix it, pay careful attention to the path here:
Open up the fallout.ini file in: My Documents\My Games\Fallout3
Find the line:
change it to:
Add another line after it and insert:
This will limit the game to 2 cores and prevent the engine bug from causing the game to freeze.
Download and install the latest klite codec pack from here:http://download.cnet.com/K-Lite-Mega-Codec-Pack/3000-13632_4-10794603.html
Did those two things and this game has played perfect since. And I absolutely LOVE this game. It's like a combination of Bioshock and STALKER, but with a better story and more imaginative monsters and environment.