Solved .. I think. It was apparently my OC of just 950 core. I dropped to 900 and have had both mining contently for more than an hour. Weird that Furmark, Shogun 2, and Haven don't reveal this. MW2 had driver crashes at 960-ish, so I've just dropped down to 900 again. My hash rates have dropped by ~30-40 on each card though :\. I'll try to get away with 930 tomorrow. I need to mine to make up for all the lost time.
Believe it or not I was still convinced this was hardware. ;-) Applications, even ones designed to stress the GPU do not load your system identically. Even with same core temp and the same % load. For example a complex instruction like a BFI_INT might break under higher clocks/thermals because it's only just barely meeting the timing requirements under normal clocks/thermals. A benchmark is not guaranteed to execute every low-level instruction a card can perform (and is likely not going to execute ones that are hardware specific).