This is a common issue with Outlook and "Leave Messages on Server". The biggest issue arises when Outlook cannot download all messages from the server within the specified time-out time (can be set in account properties). It then starts to redownload all messages again (even if it did receive some messages the first try, which can be seen in the Inbox) and it usually ends up in a continous loop untill something gives. If there is a poorly written AV, AS or proxy in the loop or some peculiar network issue, things tend to go sour as well at some point.

My advice would be to use POP3 and Outlook's "Leave Message on Server" only with extreme caution, try to get MS to finally fix their bugs or switch to another mail-app.