Hello,
since yesterday New Vegas runs like crap without any apprehensive reason. Before it ran 100% smooth but now it's like it's capped at a very low framerate with approximately something between 10 to 15 FPS. Other (also Steam) games still run fine. I just have no explanation for this and would appreciate any help. Has steam changed anything - does anyone of you experience something similar - cause it came out of nothing and I have no earthly clue what I could have changed in my system so that this happened?

Here's what I tried so far without success:
1) Stuff I changed in the Fallout.ini:
added bUseThreadedAI=1 and iNumHWThreads=2
Changed iFPSClamp=0 to iFPSClamp=60

2) tried the new vegas stutter remover from NV Nexus.

3) reinstalled New Vegas

4) deleted Fallout.ini and FalloutPrefs.ini

5) tried with and without the custom (ATI) D3D9.dll from NV Nexus

My System:
WinXP Pro SP3
AMD X2 4450e
3GB RAM
ATI RADEON HD 4870 1GB, Cat. 11.11

UPDATE: Tried some other stuff that didn't work:
changed the bUseBackgroundFileLoader=0 to 1 in the Fallout_Default.ini
I verified the file integrity and defragmented the cache files.

UPDATE2: I deactivated sound, disabled all mods, switched to lowest settings and deactivated the steam community ingame - no change in framerate.
Also I forgot to mention that the mouse already stutters in the main menu.

UPDATE3: A catalyst driver rollback to 11.9 didn't work.

Are you using any mods?

Might want to try deleting all the .ini files too.
C:\Users\\Documents\My Games\FalloutNV\fallout.ini and falloutprefs <= delete launcher will redetect video settings
C:\Program Files (x86)\Steam\steamapps\common\fallout new vegas\fallout_default.ini <= delete and then reverify game cache.

Game reinstall? Backup saves then make sure to manually remove the fallout new vegas folder after you delete local content in steam.

Thanks for the answer!
Yup, quite a few. I'll try as you suggested and install it clean this time. I already deleted the ini files as mentioned above but that didn't do anything. The only mod I tried out a few days ago was some Depth of field enhancement mod - but it ran absolutely fine with it and I uninstalled it when I didn't see much of an improvement so I don't think it's related to this or any other mod. But since PC stuff doesn't always make logical sense I guess it doesn't hurt trying.

Thats the thing about mods. Its not as easy as saying I disabled them and it didnt work. You cant always disable them by unchecking them.
You have to disable archive invalidated and revert ini changes too. Your stuttering could be any of those mods. I recommend what I usually do
is revert to clean new vegas and see if the stuttering occurs. Then add mods.

Okay that didn't work - still the same stuttering with a clean installation.
But, when I just saw the introduction video it was sort of messed up - it freezes except for a "boxy" part in the upper left corner which plays onward. So I think that this might indicate a codec problem. I don't know how, since I didn't install any codecs or codec packs manually but maybe one of the games I bought last weekend on steam installed one automatically and "forgot" to mention it. Now that I think about it, the stuttering occured just after I played Sam & Max Season 3. But it could also be Deadspace 2 or Limbo. I think I played NV the last time smoothly just before I installed Deadspace 2.
Is that possible and, if so, do you have any ideas what I could do against it? I think it would help, if I knew exactly which codecs and/or runtime Environments (incl. Version numbers) New Vegas needs to run smoothly.
Edit: I already put the FalloutNV.exe in the FFDShow audio and video exceptions so that shouldn't be it.