This happens to me on fallout new vegas, and some other Bethesda games. Try installing it to your HDD. If it still goes on then it may be an internet issue (doubt it) or just a flaw in the game. I haven't played Fallout 3 in a while, so Idunno.
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I had a similar problem when I would play Elder Scrolls Oblivion, try following the instructions in this guide to clear your system Cache. When games as large as Fallout or Oblivion are running, they save field data in a temp file on your harddrive to allow
for faster loading. Sometimes this file becomes to large and causes the game to freeze due to it trying to read all the data. See if this doesn't work for you.