iPhone 3GS keeps downloading full contents of Inbox

Each time I access my Exchange 2007 Inbox folder, iPhone 3GS keeps downloading the full contents of the Inbox (as per settings 50 items, 2 weeks etc.), removing/erasing the original screen contents, so it takes ages to "update" while it downloads all the mails again. It's not happening with the Sent Items folder which is also synched. All the mails download correctly, contacts including GAL are fine but I have noticed that it occasionally does the same thing with calendar entries on day view... Also, if I open the inbox and scroll up/down while it is "updating" it does not download all the mails... Any suggestions it's driving me nuts!?!