* Does this only happen when you have 3 grids or does it happen even with one ?

* Check your browser settings and see if the url (which passes the data in) has a cookie having a expiring data, or cache setting.

check your HTTP headers and see if there's an expiry date associated with the URL your data comes from. Normally this only happens with js or image files, but webservers can be configured to output a header with an expiry date for any url extension treating a service as though it were a file. ie with my webserver I can confgure it so that any url ending in .json will output an expiry date for that component.

http://yuiblog.com/blog/2007/01/04/performance-research-part-2/

When the browser saves a component in its cache, it also saves the Expires and Last Modified values. Specifying an Expires date in the past forces the browser to request the image every time the page is viewed (with a few exceptions, such as when users click the browser’s “back” button to return to a page). If the image is already in the browser’s cache and is being re-requested, the browser will pass the Last-Modified date in the request header. This is called a conditional GET request and if the image has not been modified, the server will return a 304 Not Modified response.