Memory Speed & Underclocking

[Intention]

Currently I have 2x Sapphire Radeon HD6950's (2GB versions).  I used to be able to underclock the memory to around 300Mhz for mining purposes but now since I had a card die due to unforeseen circumstances I replaced one with a Sapphire Radeon HD6950 DIRT3 edition.  I have no idea if its the new card or if I had updated the drivers to 11.7 (.6b) that my computer now locks up every time I attempt to under-clock the memory to the 300-400Mhz range.

Any ideas why?

Also on an unrelated note Sapphire has terrible, terrible RMA support. eVGA ftw.

[Supercrit]

Some cards just can't run at such low memory speeds, I had a 4850 that freezes every time if it goes close to 400mhz memory speed

[Intention]

Some cards just can't run at such low memory speeds, I had a 4850 that freezes every time if it goes close to 400mhz memory speed

Must be the Dirt3 edition then, I just had the regular kind before - kind of sucks since I'd like it running cooler but the dual fan is a trade off I suppose.  I'll have to fiddle with it until I find a spot that's low and won't lock up, thanks!