I've just noticed what may be a driver bug. I have 3 cards: the top one and the second from the top one are 6970's, and the last one (at the very bottom) is Nvidia 450. The Nvidia card is used for display while the two Radeon are used for computing (they don't have monitors).
The problem is that I must attach a monitor to the top Radeon to have it compute at full speed. Without the monitor, the core/memory clock is always 250/150 MHz, and the performance is rediculous. The second Radeon and the third Nvidia cards are fine -- clocks go up when the cards compute and go down when the cards are idle.
I'm using win7 with the latest Catalyst. Do you have ideas?
PS When I unplug the monitor from the top card everything stays normal, i.e. the card's clock rates change depending on the load. The problem seems to be related with a sort of initialization.