I may have figured out my particular issue, however I haven't had much time to test it out.
One specific thing I didn't post above was that I also have a Bigfoot Networks Killer 2100 card for networking. A bit extraneous, I admit, but I wanted to try it out. In any case, it comes with a QoS software that monitors PC CPU usage, as well as network traffic. I didn't think about it before considering that I've had the card for quite some time, and I recycled it from an older machine which ran a E8400 Core 2 Duo, on which CS:S was running absolutely fine.
Now, last night, I had an epiphany, and I disabled the software in my task tray, and I ran the game for a bit. Now, I also run Tomato Firmware on a WRT54GL that has it's own QoS that's presumably much better since it will control all the computers on the network, rather than just what's coming into my gaming machine. After disabling the Bigfoot software, I played about 20 minutes last night, and there was no noticeable stutter whatsoever. Now, I'm not sure that it's completely gone, since I'll have to test a little more, but that may be what was afflicting my machine. But, again, I had this software on two machines now, and the issue has only manifested itself rather recently.
So I'm going to try testing in a little bit for a bit longer and see if the problem's gone and I'll post back.
But see if you have any resource monitoring software running in the background. That may be the culprit.