I'm having the same trouble... Was thinking it might be my motherboard/CPU/RAM since it's an older setup with only 2GB of RAM. I'm hitting a max of 400kH/s but even that seems unstable.
I bought two identical cards at the same time and they both behave the same. While trying to tune one of the 7970's received the error below when trying to increase TC to 10240 from 8192. This was with an intensity of 13 and autofan. Was just following the tuning guide from
https://github.com/ckolivas/cgminer/blob/master/SCRYPT-README Anyone know what LG is? I've never seen a reference to that setting before. Is buffer memory referring to the card's RAM or the system RAM?
[2013-04-10 03:21:52] Maximum buffer memory device 0 supports says 536870912
[2013-04-10 03:21:52] Your scrypt settings come to 671088640
[2013-04-10 03:21:52] Error -61: clCreateBuffer (padbuffer8), decrease TC or increase LG
Thanks for the help, hope we can figure this out...