Gizzy -
I see this when I have several controls calculated within the report, or sometimes with complex background queries. Mine usually loads when I scroll down, but the initial controls apparently have to repopulate when I scroll back up. I'm assuming this is because it doesn't load all data at once in report view.
I've started setting my reports to open in print preview, this way it calculates all controls on load. I haven't had this problem with print preview view yet.
-Tyler