I tested this

<ISWebGrid:WebGrid ID="WebGrid1" DefaultStyleMode="Elegant" Width="100%" Height="100%"
        UseDefaultStyle="True" runat="server" DataSourceID="objDs1" >   

.....

and in the container panels i set fixed dimensions.

The container WebPart has fixed width and height.

The issue is always present, the new fact is that cpu usage is lowered to 80-90% but it is always too much.

Do you have any other suggestions?

thank you

More infos:

i'm able to use the webgrid in sharepoint designer setting trust level to Full, but when i try to see tha aspx page that holds the webgrid control i get the same error.

I also have a js error:

Webpage error details

User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; InfoPath.2; .NET4.0C; .NET4.0E)
Timestamp: Mon, 21 Dec 2009 13:47:42 UTC

Message: 'null' is null or not an object
Line: 9
Char: 34130
Code: 0
URI: http://vmoss2007/ISRes.axd?F/ISCore.js/305000750

do you understand what i miss?

the axd mapping should be good because if i type

http://server/site/ISRes.axd?G/wg_filter.gif

i correctly get an image