Overclocking Radeon HD 5830

[MystiQla]

 I just setup Radeon HD 5830 card to mine for me and I'm using MSI Afterburner to overclock it.
What's the set up to get it to highest khash/s ?
It's acting pretty strange now, as it is jumping from ~700 khash/s , to around 400 khas/s, ~6000 khash/s
Any help appreciated!