I also have 2 XFX 6950s which are showing weird behavior, although it is consistent between the two cards. Ram speed is also key to speed on these cards. After a lot of experiments I settled for 840/230 which produces 352 MH/s each. Increasing ram speed decreases performance (oddly enough), and increasing core speed does *not* increase performance (even more strange).

Also they do not allow me to change ram speed on the fly *at all*. For each change in ram speed I needed to re-flash the cards.

During the experiments I also noticed that contrary to my other cards the XFX 6950s will not lock up the system if they get to hot. They will pause for a moment, then restart. (I would not recommend running them like that, it happens at like 102°C)