This problem materialized a few months ago and I know of no changes that would explain what has happened. I have a Linkys wireless G router (WRT 54g v5) and I'm using Norton Internet Security.
Downloads stall, after starting. For instance, app updates downloaded through iTunes start, then the progress bar stops moving. I can "force" the download by pausing and then resuming. I have the same trouble downloading other items, but the Norton updates seem to be coming through.
Edited by consiglieri, 22 January 2011 - 01:16 PM.