I've been searchin for days for a solution - posting here is my only remaining option.

Bought 2 weeks ago Fallout 3 GOTY via steam and played it for some hours - everything worked fine.
Then I traveled to Meresti Trainyard. Going from here in North direction the game crashes as soon as I reach the fence with the well-known Windows error message "Fallout 3 Has Stopped Working...".
It happens every time at the exact same position. Meanwhile I detected some other areas with the same problem: Agathas house surroundings an all the North West Area of it; South East area of Megaton; etc...
It's like I'm not allowed to cross an invisible line.
Except of this the game works fine.

Maybe somebody with the same problem? Anybody here who can help?

My PC:
Intel i5-2500k
8 GB Ram
(an old dusty) NVidia GTS 250
Win7 64

I do not have installed any mods.
Also tried to reinstall the game and loads of tips I found in the web - nothing helped...

Thank you in advance!