Perhaps a solution:
today I installed some useful (i hope!) mods. Now the freezing of the game seems not to happen again. (But this is just a first impression. I had’nt the time to test the game longer than ten minutes per session, but the crashing/freezing didn’t happen again.) So I guess, one of the following mods helped, to stabilize the game. If you have the same problems like me, perhaps the following links will help:
(complete links seems to be deleted, so go to http://www.nexusmods.com/ and paste the following adresses)
– Purge cell buffers: /fallout3/mods/7817/?
– Games for Windows live disabler: /fallout3/mods/1086/?
– 3GB Enabler (4GB for 64 bit): /fallout3/mods/6510/?
– UPDATED Unofficial Fallout 3 Patch: /fallout3/mods/19122/?