Set your 2D clocks to 400/900,here is how:
1_Open CCC and choose Overdrive tab,unlock the settings(by clicking on the " Gold Key" icon)
2_Go top options,profile and choose profile manager.
3_In the composition tab,choose "The following settings" and in the "ATI Radeon HD 5900 series"make sure only ATI overdrive is checked.
Then enter a profile name,then save and close it(Do not activate).
4_Go to C:\Users\Your UserName\AppData\Local\ATI\ACE\Profiles (Its hidden so make sure you unlock hidden files and folders)
5_Open the profile with wordpad and do this edits:
<Property name="Want_0" value="40000" /> (default is 15700)
<Property name="Want_0" value="90000" /> (default is 30000)
and for the second card:
<Property name="Want_0" value="40000" />
<Property name="Want_0" value="90000" />
Save the changes and close.
6_Open CCC,in options tab go to profiles,profile manager and choose the profile that you saved before and then click on Activate and close.
Now your cards will idle @ 400/900 and you should not experience any more problems in 2D mode.