Ubuntu :: Download Managers Can't Resume Downloads - Errors?

I have tried so many download managers it ain't funny, but there's one thing they all don't seem to know how to do well, and that's resume problematic downloads. I have used both fast and slow Internet connections on 3 continents (currently I'm on a slow and intermittent one as I live in Burma) so speed doesn't seem to be what causes the problem. And I download from many, many different sites, so it's not site specific either. The problem is as follows. I'm downloading a file from a server that supports resuming downloads but something goes wrong with the download and the download manager reports an error, like "size mismatch", or "timeout error".

What I really, really want to know, is why can't these managers resume downloads that experience these errors? Why can't they set markers during the download process (say every 5MB) so that if a problem like a size mismatch, a connection dropout, or a timeout occurs, then it can backtrack only slightly and then resume? I download a couple of shows from revision3 each week and more often than I care to remember they've failed at around 70% of a 300MB+ download and merrily just start downloading all over again. Another frustrating thing is they ditch the file that didn't finish, so I can't even watch the part of the show I had. I live with it, though it's very frustrating, but I wonder why these download managers aren't more intelligent.