ModalPopup with database records becomes blank

Hi,

I've an UpdatePanel triggered by a button. On button click a ModalPopup opens and a database call fills a GridView inside it. Sometimes the records inside the popup suddendly disappear after about 1 second leaving a blank and clear window. What happens?

Maybe I have to show the popup only when the data are completely transferred...I don't know...

Thanks for any help!
...