My Sapphire dual-x does the same thing. If you want, get an overclock tool and set it to 951mhz and it will stay there all the time if you are that worried.
What it does is uses the 850mhz on parts where it isn't as demanding and doesn't need the boost, but when it gets more demanding, it uses the boost technology to help it out with the demanding parts.
I upped the clock speed to 951mhz using triXX and it still cycles between 850 and 951. It seems to cycle between the clock speeds regardless of the load, it stays at 950 for no more than a second or so. Should I turn the boost feature off and manually turn it up to 950mhz?