Have you any mods installed? If so remove them all and start a clean save.
Download the
unofficial patch (IRC there's some other crash fixing patches on that site too), and the
script extender.
Backup and delete all your saves and start a new game.
Fallout 3 (and every other game on that engine) is unstable as hell, though you shouldn't be experiencing too many issues with an unmodded game. If you do have mods installed its a simple matter of just removing them each one by one and seeing if the game works. From what you're discribing though, ie playing it for a time before the crash occurs, it could be down to a few things. Does it happen in a particular area for one? It could be a dodgy script in one area, or an enemy spawning incorrectly, or umpteen other things. =P