Every time i'm about to enter combat, game starts to stutter heavily and it lasts for 4-5 seconds then goes back normal.I got no clue what could cause it. My fps is good (40+) and i did those recommended ini tweaks (http://www.tweakguides.com/Fallout3_1.html)
Here's my load order:
[X] Fallout3.esm
[X] Anchorage.esm
[X] ThePitt.esm
[X] BrokenSteel.esm
[X] PointLookout.esm
[X] Zeta.esm
[X] CALIBR.ESM
[X] xCALIBR.esm
[X] FOOK2 - Main.ESM
[X] FOOK2 - [DIK] DLC Improvement Kit.ESM
[X] FOOK2 - [[censored]] World And Neighbourhood Kit.esm
[X] Lings.esm
[X] LingsDLC.esm
[X] DarNifiedUIF3.esp
[X] CASM.esp
[X] xCALIBRmunitions.esp
[X] xCALIBRmunitions_FOSEnhancements.esp
[X] Mantis07Reflex.esp
[X] UndergroundHideout.esp
[X] kikaiEquipment.esp
[X] 1PipboyPDA.esp
[X] FOOK2 - Main.esp
[X] FOOK2 - [DIK] DLC Improvement Kit.esp
[X] FOOK2 - [EVE] Energy Visuals Enhanced.esp
[X] FOOK2 - Main [Hotfix].esp
[X] FOOK2 - Mothership Zeta.esp
[X] FOOK2 - DIK [Hotfix].esp
[X] xCALIBR_override.esp
[X] mpatch.esp


PC specs:
AMD Athlon 64 x2 6400+
2GB ram
Ati Radeon HD3870 512
500gb HDD
Win7 64-bit Ultimate

Game settings:
1680x1050
2x AA
8x AF
no bloom,no hdr,everything else on max except shadows turned off

Could be 1 of 2 things (Or both) 1) I had same problem i also thought it was Video lag turned out to be audio lag.So i sey my speaker config (In comtrol panel) to headphones.And lowered Hardware Accel to standared. 2) Could be others cells loading same time you are in combat. I have just about the say PC specs as you (Execpt my ATI Radeon is 1 gig also HD and i only have a 80gig HDD and running Xp.With a 42" Monitor(T.V really)

I'll try changing audio settings and hope it works .
btw stutter only happens in outside cells,indoors everything works perfectly.