Don't "alt tab".
Get ATI Tray Tools, and you can make a graph, showing what the clock speed and when. The program will also allow you to decide if you want it to kill the 2D/3D speeds, and make the card run at rated speeds at all time.
Also, this is normal, ATI cards have that to save power, and produce less heat. In 3D application, the speeds are as rated.
I recommend you try uninstalling the Catalyst, use the latest drivers. Instead of the Catalyst run ATI Tray Tools.http://www.guru3d.com/article/atitraytools/189/