As the post title suggests, I'm having a pretty significant drop in FPS in certain interior areas, like the REPCONN facility, Helios 1, and Vault 3n. It's bad enough to make the game nearly unplayable. I was having MASSIVE stuttering and lag before applying the .dll, but now it seems to be a lot better, except in these cursed areas (that I've played so far). Nothing I do in these places, and no amount of settings tweaking will raise performance in the slightest. It doesn't make any sense, other places that appera to have just as many NPCs or complex geometry run way better.

I have dual Xeon E5420s quad cores, a DX5400xs mobo, a Geforce GTX 480 running the 260.99 drivers, a separate 8800 GTS for PhysX, triple monitors, Windows 7 64x, and 8GB RAM. No overclocking of any kind, everything is at factory defaults. Temperature monitors are great, as I have a plenty of cooling fans.

I run Fallout 3, and every other game I throw at this machine, like a dream. Is anyone experiencing this specific problem, and did anybody have any success in fixing it?

well the xeons are not fully utilized im sure in this game it does help your gpus out though at least in communicating to each other. As for your issue the dll fix that you put into the directory is pretty much an old directx 9 file that makes the game think your running 7900 cards so I am sure not all video ram and so on is being utilized. I dont have problems w/ interiors on my GTX 480's just an idea take out or disable your 8800 GTS and see what happens.

I have the same problem as the OP. Except I'm on a modest laptop rig compared to his. (Vaio F11, Core-i7, Nvidia GT 330M 1GB, 6GB of RAM and Win 7 x64. I get some pretty annoying stuttering when roaming the open waste only when it's loading objects off in the distance and pretty bad stuttering in the same buildings as you and a few more that I can't remember. My game usually performs its best in most buildings though. I'm just eagerly awaiting the promised update for this week that's supposed to improve performance for Nvidia users so we'll see I guess...

Yep same for me on my laptop (Games fine on my desktop build tho). But in my laptop i get horrible stuttering when moving around , sometimes slow downs when looking at some bushes, And i also have crazy slowdowns indoors when looking at a direction, If i look away its fine again. These slow downs dont even make sense Because most of the time when they happen im just looking at empty space, theres nothing there that could possibly slow the game down.

I experience the exact same performance problems. When I'm roaming about in open spaces with what should be much more demanding environments, I get very good fps - mostly 60fps. When I get inside some buildings or even just looking up close at a wall and strafing side to side the game is unbearably laggy. I get headaches because of it so I cannot play for long periods of time. It really makes no sense at all.