Interesting; mine dropped a bit down to 2.1Th/s over the last 6 days. I am still on 675MHz. It started at almost 2.5Th/s. Did anyone else experience such a little bit of drop-off after the first few hours of operation?
My only theory right now is that they get slower as the heat up and the heatsink becomes fully soaked. Not sure about that, though.
I had the same experience. With stock voltage, I tried various freq. settings from 500 MHz up to 725 MHz. At the beginning of the run, cgminer reports up to ~2.5 TH, then, after 30min or 1 hour it slowly drops the hash rate.
Currently, at 725 MHz (which seems the maximum, increasing it, the HW errors raising as well to an unacceptable ratio), the miner is dropping down to ~2.190 TH.
Also tried to increase the voltage knob from 12 o'clock to 3 o'clock in small steps, to no avail. So I turned it back to the factory setting.
The next step will be to measure temps and increase airflow if necessary.