Artifacts with Direct3D 7 when OCing

For some strange reason I get artifacts in Direct3D 7 when OCing the memory even a tiny bit. My OCs are all 100% stable by any standards and work well with Direct3D 8 and 9 (as evident in the DXDIAG test).

This disturbs me as I'm playing World of Warcraft now, and it uses the Direct3D 7 interfaces, so my screen flicker all the time if I OC my memory.

Anybody knows what is the source of this Direct3D 7 specific problem, and how can I fix it?

Thanks