Please note that custom created controls are not supported even if they are inherited directly from RadControls.
Try using a regular <telerik:RadGrid> declaration and if the issue still remains, please open a support ticket to send us a sample runnable application demonstrating the problematic appearance or provide us the exact steps to reproduce the issue. Thus, we will be able to further analyze the issue and provide a proper solution.
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 now.