bizzarre issue

My system specs : MSI 975X platinum motherboard.
core2duo 1.86 ghz CPU, geforce 7600 GT, 1.5 GB ram, 650 W PSU, realtek HD onboard audio. XP proffesional up to date ( all hotfixes SP2, e.t.c) all up to date drivers.

Now heres the deal, when i play older games, more than a couple of years old, 10-15 minutes into gameplay the game will hang, and then the sound loops continiously, i have to alt+tab out and back into the game, and within 2 minutes its doing it again.
It dosnt do this on games like oblivion or counterstrike:source which run very nice.

Sounds like a dualcore issue right? i installed the windows hotfix, enabled it, still does it, i tried turning off Nvidias multithreaded optimisation, still does it, i tried manually setting affinity, still does it.

the games it does it on are counter-strike 1.6, warhammer 40k dawn of war, freelancer, (freelancer does it very bad ) and a few other older games ive played.

Now seeing as ive tried every dualcore related fix ive seen bandied about, i then considered my onboard soundcard, which are usually culprits, so i disabled it in the bios, and the game still hung, i tested each memory stick on its own, and in different configurations and ran memtest.... nothing.
i have no overclocks, im not overheating, the PSU wattage is fine for the mobo. i dont have any viruses or spyware.
i did a clean reinstall of windows, it still did it, i tried older Nvidia drivers, beta nvidia drivers....
im at my wits end, surely it must be a hardware issue if a windows reinstall didnt fix anything... but why does it only do it on less intensive games?

someone please give me some ideas to try. :?