I encountered this problem running windowed games. It would not kick my HD4870X2 into full speed unless I was running a full-screen game. You can disable the power-saving crap by editing the profile file and changing some settings. My card runs at max speed 100% of the time now.
Edit:
Win7: C:\Users\<user>\AppData\Local\ATI\ACE\Profiles\nop owersaving.xml
I changed all the "Want_0" values to be the same as the "Want_1" values. You could also try playing with:
<Feature name="ForceHigh3DClocks">
<Property name="ForceHigh3DClocks" value="Disable" />
</Feature>
P.S. There's a chance you could fry your card, be careful what you change.