So to begin with, I'm a pretty technical guy. The issue doesn't appear to be driver related, nor is it related to the performance of the hardware in the system, which exceeds the system requirements of the game. That said, I have been experiencing slowdowns in the game that seem to be more a function of time than anything. After 5-10 minutes, the game will slow from silky smooth to 2fps or less. This will last for approximately 1-2 minutes, then revert to normal. There doesn't seem to be any rhyme or reason, aside from the length of time I've been playing. The only new software I have installed since this happened are windows updates related to HID-compliant input device (mouse). I have added no new hardware, aside from moving my system into a newer, more roomy and breezy case. System specs are below:

OS: Windows 7 Ultimate x64 SP1
Proc: AMD Athlon II 455 x3 @ 3.1GHz (stock)
RAM: 8GB (2x4GB) g.skill Ripjaws X Series PC3 12800 (O.C.)
GPU: PNY nVidia geForce 9400 GT 512MB
Hard drive: Seagate Barracuda 7.2k RPM 1 TB

The problem occurs every time I play, and seems to disappear when I got to the ESC menu, but comes right back once I leave the menu. It also continues to occur inside, outside, and when looking at the pip boy.

nVidia geForce 9400 GT 512MB may be your problem, the same thing used to happen to me when i played fallout 3 id get ten minutes play with no problem then slowdown then it would pick up again slowdown then maybe crash. replaced my 8600gts with a gtx 460 and that crap didnt happen anymore also i havent experience any slowdown in NV apart from the norm when its loading towns and such.

I had initially thought it was the GPU as well, but the game worked flawlessly until about a week ago. Perhaps it's just something loose. Since no one else has mentioned the problem, I assume it may have something to do with my system.