Sometimes this problem is caused by background windows programs, I would recommend not having things running in the background while playing.
There are also various things that can be fixed by altering/deleting a file called "ddraw.ini" that is found in the Fallout directory. I don't have any details on this I'm afraid, but I would recommend checking out http://www.nma-fallout.com/ and http://www.rpgcodex.com/ since they are likely to have threads with information you could search for in their forums.