This may have been discussed in the past but I've been searching the forum for 2 days and haven't found anything.
I'm currently running a Diamond 6970 http://www.newegg.com/Product/Product.aspx?Item=N82E16814103182&cm_re=6970-_-14-103-182-_-Product
and having a trouble getting the card to Clock over 900. The card is running fine at 900/300 around 370 GHash/sec. As soon as I move the clock over 900 (even to just 901) it drops to around 330 Ghash, and drops even further the higher it's clocked. The card appears to be stable at higher clocks except for the GHash issue. Aticonfig reports that the overclocking is running correctly for the core, but memory is displaying at around 150. I flashed the BIOS because I was unable to get the memory below 1275 with the factory BIOS, but even with the stock BIOS the same problem was happening over 900 core clock.
This is all running on Ubuntu 11.04, Catalyst 11.6, SDK 2.4, and phoenix (latest phatk kernel). I'm running several other 58XX cards in the same system and have not run into any problems. Any idea what is happening here?