Often it is due to an overlly large message on server. Try using File menu, Selective download to see if you can identify the overly large message and maybe to remove it. Does your ISP also offer a webmail interface you could use to try to view the overly large message in the queue?

Status shows 'downloading message x of n ... %' gets to 3 or 4 messages then hangs.

Any ideas? 

Any av scanning in place?