Fallout 3 lags like crazy on my system but no other game does. WTF?

Playing Fallout 3 i get to a certain point early in the game and it starts to lag like crazy. I'm talking 1-2 FPS. It's unplayable. Up until that point the game runs fine. Other games such as Black Ops, Crysis, Crysis 2, GTA4 run beautifully. No other game does this. No scans or anything like that get... show more Playing Fallout 3 i get to a certain point early in the game and it starts to lag like crazy. I'm talking 1-2 FPS. It's unplayable. Up until that point the game runs fine.
Other games such as Black Ops, Crysis, Crysis 2, GTA4 run beautifully. No other game does this.
No scans or anything like that get kicked off during the lag.
Can anyone tell me why?

My system:
Core2 Quad @2.66GHz
HD6970
4GB of DDR2
Velociraptor 10,000 RPM HDD
Update: Forgot to add. I'm running the very latest drivers direct from AMD's site.
Update 2: Sorry about the TD's. It seems i have a TD fairy following me.
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