When you are not on 3d applications your gpu downclocks in order to conserve power. Thats why clocks shoot down to 150/300 respectively. I actually had this same problem with flickering on my computer. What video card are you using, and what internet browser is it happening on? If it's an amd card and you are using mozilla firefox it may be the video card driver. If you are using firefox, try using the latest version of firefox or disabling hardware acceleration in firefox. If that doesnt help try using the latest driver or using a previous one.
There is a way to set 2d clocks manually, but i forget it. But u can download iTurbo from HIS and it allows to disable 2d clocks.