I am having the exact same problem with my HD 4870X2, also a Sapphire brand. I know why, just not how to fix it yet: When a core is not being used, it downclocks itself to save power, however sometimes certain programs, such as the bitcoin miners, cannot trigger it to go back to full speed. I suspect using a third party program to lock the card at full speed would work; I don't know of any that work, but I'm going to try MSI Afterburner per mewantsbitcoins' suggestion!
Also, just fyi, if you have two 4870X2s like I do, only the first core stays overclocked and the other three are all stuck at around 54 Mh/s.
EDIT: MSI Afterburner isn't doing it, but I do enjoy the dynamic fan control feature