I'm running a MSI Neo 2 Platinum with an Athlon 64 3000 @ 2.4GHz and an X800 XT mildy overclocked, for detailed details

it's in my sig.
I loaded Steam last night to play some CS:S online and Steam tells me that my video drivers are out of date (Catalyst 5.1), so I think yeah, fair enough, I haven't updated my video drivers for a while so I go off and download 5.5.
I removed the old drivers and installed the new ones, everything looks fine, I go back into CS:S and after about 10 minutes it hangs!
I reboot and try again, again 10 minutes or so it hangs again.
I removed Catalyst 5.5 and try Catalyst 5.4, but the same thing happens, but I noticed after the 3rd or 4th hang that it seems to happen if I go into the console or when menus are on the screen, but not exclusively.
I've tried returning the X800 to stock speeds, updating the motherboard BIOS to 1.8, running the CPU at stock, reinstalling the nVidia chipset drivers, but I can't think of what else to try. Plus, prior to changing the drivers everything was fine.
Any ideas what's going on here? Also can you tell me where I can get Catalyst 5.1 from, to see if it really is a driver problem?
SC