one of the things you might try is eliminating some of the variables ...
what browser are you using? if it's IE, try Firefox ... if it's Firefox, try IE ... no idea if the problem is browser related but it might be worth a try ...
are you having the same problem sending lengthy emails? if so, the problem would seem to be one of transmission (e.g. packet sizes) or perhaps available memory ... if email works for large emails, again, it sounds more browser related ...
also, it might be a good idea to run a test with only your browser running (i.e. no other programs) ... maybe, just for one DU post, shut off your anti-virus program and your firewall ... see if that helps ... make sure you turn them back on though ...
overall, it sounds like you're "timing out" before the post transaction completes (i.e. when you're "uploading") ... it's hard to imagine a longer post would cause that to happen ... to run an "upload" test on your upload speed, try this website:
http://www.toledorocket.com/perftest/uploadtest/fileselect.aspwish i knew more ... i really have no idea ... test what you can until others respond ... speaking of which, it might be helpful to provide more info ... e.g. PC or Mac? Windows XP?