Hi Henrik Næss,
As we described about a year ago, the major problem comes not from any of the telerik controls, but from other /toolbox/ controls that are added in the RadTabStrip
content panel. Unfortunatelly, there is no straight forward solution to this problem.
You can take a look at the famous windows forms FAQ that describes a possible solution to this problem here: http://www.syncfusion.com/FAQ/WindowsForms/FAQ_c73c.aspx#q1039q
This solution should work not only for the Panel but for the GroupBox control for example.
As to our future release, we do plan developing "Rad" versions of most of the toolbox controls, so placing RadControls inside TabStrip Content panel or RibbonBar should not cause any flicker.
Thank you for writing. Contact us again if you have additional questions.
the Telerik team