If Shareaza keeps dropping the same source chances are that source is sending a "404 File not found" to the request.
Check in the system window when this is happening as you re-ad the source & it gets dropped again.
That can be caused by that source removing the file from the library or unsharing it within the library.
It might be still showing in results from that source because it hasn't updated the shared list with the Hubs.
Once that happens that source should no longer be shown for that file when a new search is performed for the file, well I think that's sort of how it works anyway.
It could also be as you mentioned a firewall or uploading problem by that particular source.
Couple of things to try, search by SHA1 for the file, right click it in the downloads window > advanced > copy URI.
Open a new search window, paste the Sha1 in the search box & click search.
Changing some of the Advanced Settings may or may not help.
Downloads.MaxAllowedFailures - increase
Downloads.MinSources - change to 5
Downloads.NeverDrop - change to True
Downloads.StarveGiveUp - increase
Downloads.StarveTimeout - increase