Fallout 3 keeps leaving the game with no warning no what so ever, it will go to a black screen then leaves, or it just freezes.

I think it may have something to do with my graphics card.

I have an ATI Radeon HD 4800 seires.

How hot is the card getting when in game? And what CPU do you have?

not very hot.

And i have a Intel Pentium I5 Proccesser

Try the quadcore fix.
Open up the .ini file located in My Documents>My Games>Fallout 3. *Do not alter the one in the actual game directory labeled Fallout_Default_ini*
In the FALLOUT.ini find the line

bUseThreadedAI=0
change the 0 to a 1

then below it add the line

iNumHWThreads=2

Still no luck.
It comes up with an error message now instead.