Unfortunately from your description I am not able to determine the exact cause of the issue you are currently facing and we have not been contacted with regards to a similar problem before.
To identify whether the problem is DNN-related or not, can you please check whether moving the grid outside of the DotNetNuke environment makes a difference? Additionally, I will appreciate if you provide a live url where the erratic behavior can be observed. Thus I will do everything possible to provide more definite answer/fix.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed