It's not a problem, it's simply a power-saving thing that ATI has in all of their drivers. What you are looking at when it's at 400 is the 2d clock speeds. What you have set at 900 are the 3d clock speeds. ATI drivers detect when you are using a 3d application, and boost the clock speeds accordingly. When you are in 2d mode, it significantly lowers them in order to save power.
To see for yourself, try a program like GPU-Z, set it to update when it's in the background, and have it start logging your clock speed. Then, run a 3d game. After you've played for a while, exit out and check the log. You'll find that as soon as you started the game, your speeds jumped right up to the 900MHz you set it at.
Congrats on the 5870 by the way, it's an excellent card isn't it?