In that case I'm stumped. Unless it's a case of the observer changing the behaviour. If you are using the computer to tweak the card and then opening different apps this will affect framerate and will affect mhash/s. Leave it running for a few hours mining and see what the average mhash/s is
You know, actually, I think it may be the temperature.
It would be stable up until about 72c, and then the usage would bottom out and then go back up.
No idea why though, because that's a perfectly good temperature. This card is good up to about 90c.
This means I have two issues: Getting about 100 Mhash/s less than I should, and having the card throttle (or something) at 72c.
See here:
https://i.imgur.com/dwLM02f.pngThe top line is GPU usage, and the line below it is temperature.
You can see that the usage is maxed out until the temperature hits 70c, and then it starts dropping out.