I don't know if this is a CPU issue, or a video card issue, but I'll start in video.
Intel E8400 (3.0 ghz)
Asus P5G41T M LX
Corsair XMS 4GB (technically 3.5 because I'm still on XP 32-bit) 240-Pin DDR3 SDRAM DDR3 1600 PC3 12800
EVGA Geforce GTX 560 Ti 1024mb
Corsair TX650W PSU
Windows XP Home 32-bit
Pretty noticeable that the GPU might be getting bottlenecked by the mere dual-core. However, this is happening in games I have had no difficulty playing for a year with this hardware, and with the nature of processors, I don't believe they really degrade, so I'm just assuming it's still a video issue.
I was having a stuttering problem, where games would stutter every couple seconds. The worst of it was Batman Arkham City, which a couple months ago I played through twice on the same hardware with no difficulty. As of April, the game began having very noticeable framerate drops for maybe half a second. I found that my Nvidia settings had been reset to quality and was forcing high anti-aliasing in all of my games. So I switched them around and everything was fine. A month after this incident (starting last week), the problem seemed to return, despite any changes I've made to reset the Nvidia settings to where they were when it was working fine, or setting them to "Let 3d application decide." It's worse than ever in Arkham City, and it now even affects Source games, which I've never actually had a problem with on any PC. Source games, when uncapped, will run as high as 200+ fps, but now they're inexplicably dropping as low as 40-20 fps, again, just for half a second. It's baffling, and while I need to upgrade the CPU, I'm not exactly eager to do it over something that could easily be a video card issue.
Tl;dr: Framerate is dropping from 60+ to around 20 in very short but noticeable intervals. Not sure if it's a graphics glitch, since I've repeatedly clean installed numerous drivers, or a hardware fault.
So far I have: Clean reinstalled video drivers, from the latest 301 all the way back to 280. Installed second Windows partition on new hard drive and tried running from there. Replaced both 2GB sticks of RAM. None of this has been any help.
GPU idles at 30-40C, maybe about 60C under load. CPU cores idle/ internet browsing at 40C. Haven't found any in-game temperatures for CPU yet. I don't have a spare video card to test.
There's nothing in the background (this is a clean Windows install, from the disc, just done today, so no trial versions or fluff hidden away either), and I'm not at all comfortable with updating the BIOS. This is the same motherboard it's been for a year, and the last time I updated the BIOS on a motherboard it turned out it wasn't the problem at all (laptop wasn't properly switching between Intel and Nvidia video, so I won't find the solution to this problem in that one).
For stutter's like that, it could be an issue with a piece of hardware not necessarily a software issue. There may be a hardware interrupt. I remember having this type of problem on an XP laptop in 2004.
My only guess is that a piece of hardware is malfunctioning. Anyone else can chime in though for a solution for you.