how to analyze unreliable downloads

For weeks I have had trouble getting my computers to reliably download from the web. Browsers work fine, and my internal network also seems to work fine. But pretty much everything I download from the web comes in as corrupt. IE eihter complains that the file is corrupt, Mozilla stops the download after a few seconds and tells me that the download was interrupted, When I do get a file, the checksum is wrong, larger files (videos) cannot be run, etc. This happens on at least 3 computers connected to my network (All Windows, one Win7, one Win8, one WinServer 2011). To get to the bottom of this, I ran Wireshark for some downloads, and the logs do show some issues, but I don't know where to go from here.

The log shows the computer happily running along with lines that I think show the progress. Then suddenly black lines appear: "TCP Dup" and "TCP previous segment not captured"(how can I post the lines here?)

So something is going wrong. How do I further analyze that and, more importantly, fix that?

Thanks.