How to stop dropping the header

Hi there

 I have a problem in my page that is not easy to explain.In my page I have a header (user defined control) on the top. When I refresh the page or go to another page (which contains the same header obviously), while the page is comming on the screen, the size of the header changes. It seems it comes to the screen and then resize itslef to a more high. For setting width and height of the header and the page, I have used percentages, even when I changed it to absolute values, the problem was the same.

I know it might seems strange, but I have to fix it and couldn't find a useful way.

Thanks for your valuable help.  
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