Firstly have you got the latest patch for the game ... here ...
http://www.3dgamers.com/games/doom3/downloads/
Then make sure these lines are changed in your config ...
seta r_customHeight "1050"
seta r_customWidth "1680"
seta r_mode "-1"
seta r_aspectRatio "2"
If the config keeps changing back then you must run the ...
"Scan Hardware and Select Optimal Video Quality" in the games display setup and exit the game ...
go back into the game and play a bit ... exit ...
THEN do the changes to the config.
There's no need for the FOV change as the ... seta r_aspectRatio "2" sorts out the correct view for you and the config file deletes the FOV entries anyway.
If you really really feel that you want to alter the FOV you can do this by adding the seta g_fov "100" (or any other value) to the target in the Doom3 desktop shortcut properties...
".... \Doom 3\Doom3.exe" seta g_fov "100"
Hope this helps ...
