My Powercolor 5970 runs fine with the DD block, no lockups at all at 950/1200/1.25v.
I did have some lockups earlier (that I didn't have with my older 4870X2 card), but I just had to raise the CPU voltage a notch (1.37v - 4.4ghz i7).
Based on that, it would seem that this new 5970 is a little more sensitive to CPU O/C... or it was just a coincidence.