mysterious chugging/lagging

Hello all.

I hope this is the forum to post in, but I just joined the boards as I am in need of some help and direction with a problem with my computer.

It is a AMD Athlon XP 2800+ Barton.
It's using 2 sticks of 512 ram, and has an AGP 6800GS 256meg installed.

I've had this computer for a few years now, I think it would of least have been three.
And it's been through a few different RAM sticks, and had an ATI 9600pro installed for quite a while.

Now, I installed a 6800GS 6 months ago, in order to run newer games, as it's basically only used for that, and photoshop/illustrator for my art. And while I'm aware it's an old processor, that card allowed it to play anything available today whilst looking and running amazing.

But recently, over the last 3-4 weeks maybe, it has had been experiencing major lag that comes at random, causing any game (and i mean ANY game, from old games like jagged alliance 2 to the newest of the new, such as neverwinter nights 2, to lag to a point of running at perhaps 2-5 FPS, for at least 30 seconds at a time.

Now, the games run PERFECTLY the rest of the time, and most of them with alot of the fancy features turned on and up. And before you ask, yes, turning things down or off, from both ingame menus and the nvidia control panel does not lower or affect these 'lag/slowdown-spikes' in the SLIGHTEST.

So I took it to my local computer man and got him to do a complete format and reinstall, as I'm a lazy bastard. And it was done. I told him of this issue, and he told me he'd run 3dMark06 and 05 for a few hours each while he was working to see if anything was up.

Now, he claims he didn't see anything, but I've got the feeling that if 3dMark was running it's cheesy sequences smoothly along most of the time, he could of easily not have noticed or payed any attention to the lag-spikes, which were quite apparent when I ran 3dMark for a while at home.

Since the reboot I have tried to lower the problem by changing drivers for my GPU, lowered my CPU's clockspeed, raised the CPU's clockspeed, swapped around RAM sticks, and taken apart the whole computer and cleaned out all the dust (), along with all the obvious full-scans and virus scans and defrags etc.

I'm not computer illiterate, in the sense that I have been playing PC games as a hobby (okay, more than a hobby when I was in school, hehe, downright obsession) for maybe 10 years now, and I know how to reformat, pull apart, clean, keep clean, overclock and tweak minor things, defrag and so on, more than most who have no training or study in PC workings.

But I am self taught and my knowledge is quite biased to that only which I have needed to learn, whether that be by tweaking games or my system to run them better, or reformatting/upgrading/maintaining/networking the families other gaming or work pc's.

So to make it brief, hehe, my PC has mysteriously lost alot of performance, and it is beyond my knowledge to work out why.
Any help or direction is much appreciated, and if anyone needs any DXdiag information or anything fancy in aid of assisting me, I'll cooperate to my full capacity.

Peace brothers and sisters, thanks in advance.
-Eddy