same problem here. i experienced randomly crashing in my fallout 3 about 5 to 15 minutes sometimes 30 minutes. i did everything i could to play the game smoothly with a hope that the game wouldn't crash. i've installed the latest gfx card, latest directx, i even tried every fallout 3 patches from the earliest till the latest, i did clean reinstall and tried again. also i heard that fallout 3 was conflicting with ffdshow codec, so i unistalled my K-lite codec pack. after i uninstalled the codec i got lucky could play like 3 hours+. but then again the next two days after i played it a while, the crashing back. also i've changed my bUseThreadedAI (something in fallout3.ini file) into some correct specific number cos that was ppl said the problem is, but still the game keep randomly crashing!!!!...i've send my issues on their official website but it seems they dont care. i guess the game is only a bad product.
Fallout 3 is a very buggy game, i have both PC and Xbox versions, and the PC one is definitely the one that crashes more, I also changed the, "bUseThreadedAI=0" to "bUseThreadedAI=1" and something else, but it still won't work. I love the game, but I think it is quite buggy, and its not the computers that are bad.