I have done at least 3 playthroughs on this game on High settings, and 1920x1080 resolution at around 50 - 60 FPS. Recently I finally reformatted my computer because it badly needed it, I reinstalled all the latest drivers, and all the latest versions of my games(via Steam).

Soo last night I decided I wanna play Fallout 3, I added a few cool mods, and started a new game. Escaping Vault 101 and all that crap was fine, I ran at a constant 55 FPS. But once I got out into the wasteland, my FPS would start at a nice 50 or soo, and over the time of 20 - 30 minutes, the FPS would slowly decrease to around 20, and it would stay put there. Whats odd is, if I run the game at all lows on 640x480, it does the same EXACT thing as if I was running it on Highs on 1920x1080. Ive tried running the game vanilla with no mods, overclocking, using older driver versions that are known to work with the game. And I got nothing changed.

Its a really odd problem that I have never seen before, I searched google and came up with nothing. Ive played through Crysis 1 & 2 on this machine with max settings on both games, and they ran flawlessly, I do understand that there are pretty distinct differences between Fallout and Crysis. But there is no explanation for why I can run Crysis 2 at 50 FPS and Fallout 3 at 20.

My system specs are:

Intel Quad Core OC'd @ 3.1 GHz

NVidia GTX 260

8 Gigs of Corsair DDR2 800MHz RAM

(I can specify more if needed)

The mods I have enabled are:
Fallout3.esm
Anchorage.esm
The Pitt.esm
StreetLights.esm
BrokenSteel.esm
PointLookout.esm
Zeta.esm
Apocalypse Armory.esm
Enhanced Weather - Rain and Snow.esm
GalaxyNewsRadio100[M].esp
GalaxyNewsRadio80[M].esp
GalaxyNewsRadio60[M].esp
GalaxyNewsRadio40[M].esp
GalaxyNewsRadio20[M].esp
Apocalypse Armory.esp
Apocalypse Armory - DLC Enhancements.esp
1PipboyPDA.esp
Fellout-pipboylight.esp
Enhanced Weather - Rain and Snow in Fallout.esp
Enhanced Weather - Weather Sounds in Interiors.esp
Enhanced Weather - Sneak Bonus during Storms.esp
Fellout-Full.esp
Fellout-Anchorage.esp
Fellout-BrokenSteel.esp
Fellout-PointLookout.esp
Fellout-Zeta.esp
(This is also the order that they are loaded in as well)

Any help is appreciated.