Help! Major Graphical Corruption!

Hello.
I have a dell XPS420. No components are OC.
Specs:
Intel Quad Q9400 @2.66
4GB RAM
Radeon 3870x2
1TB HD at 7,500rpm
Vista Ultimate 32bit
My computer shipped with an ancient version of Catalyst drivers (8.5 or something like that). Since then, I have updated the drivers many, many times. The original drivers worked fine save for some minor issues with certain titles, so I upgraded to fix.
Recently I tried to play some Shader Model 3 based Directx9 games. Whereas DX10 games run fine, in many titles such as Dead Space, Empire Total War and EVE Online Apocrypha, random vertices are randomly moved around the screen, resulting in a horrific mess of multicoloured flickering triangles.
I really don't want to downgrade my drivers back. I haven't been on any SM3 games for a bit, so for all I know my problem could begin back at some only slightly newer than original drivers. Can someone please help? I assure you I have no heat, power or system stability issues.
Thanks in advance.

Option to clock the card isn't available in CCC. (no overdrive menu due to dell not using overdrive compatible power cables - think they used 2 6 pin as opposed to 6 and an 8 pin)

Driver downgrade to 8.6 (as low as I want to go) solved nothing. It also, thanks to an 'ATI HDMI Audio Driver' which Catalyst installed, meant I got no sound in every program apart from my sound card diagnostics.
Anyway can anyone suggest a solution apart from a downgrade? It happens with any games that ATI have not really ever given any attention to in their drivers. Games like Crysis in x9 work fine, although it is based on SM3. No corruption in the SM3 tests in 3DMark06. All DX10 titles look and run great. All SM2 based apps look and run great.