Yes you've explained what you're trying to do, but I'm trying to get to the why of it. You see, what you describe can be done in a simple extender or an updatepanel. The modalpopup is meant to get just one piece of information from the user and get the page to process it. You're attempting to use it as a page/control in its own right and so you are going to run into either difficulties or weird behavior. Have a look at this thread, I don't know if it helps:

http://forums.asp.net/p/1018610/1374063.aspx

But my point is that if there is logic in data being displayed, it should be on the main part of the page itself rather than in a modal popup.