So, a little digging around, I came across someone with the exact same problem with his 7870. His turned out to be a driver issue.
I would try the new 12.11 betas. If that doesn't fix it, report it to AMD and see if they have a fix for it.
I picked up a 7870 last week too and had a few performance niggles,after running 2 old school 6 year old nvidia cards the difference with the 7870 was immense but it had a slightly bogged down feel to game play.
After checking clock speeds in MSI Afterburner it showed the card fluctuating between 1050mhz and 450mhz on the core during gameplay so the the frames were up and down as was the gpu usage which was mainly around 64%,rarely in the 80's/90's,dipping down to 20% regularly too![]()
I uninstalled the 12.8 drivers and installed just the 12.6 driver (without the CCC and all the other stuff) and bingo! no fluctuating core anymore,once any 3d action is happening it shoots straight up to 1050 mhz and stays there until game over![]()
The GPU usage however still hung around at 60-20% until I increased the 'power limit' slider in Afterburner to +20% and hey presto usage is now in the 80's/90's........performance is night and day compared to how it was,settings are now cranked up and it's fantastic.
I don't know why the power limit was too low on mine but after reading about other 7870's throttling down because of the TDP limit it was worth a shot.
I would try the new 12.11 betas. If that doesn't fix it, report it to AMD and see if they have a fix for it.