Interesting.

If you were on broadband, you've described what happens when ISP's filter uploads. The first bits get through, then the filter stalls them for 60 seconds or so and the packets are lost.

You can see this on the Tools->Statistics->Bandwidth->Upstream graph pretty clearly.

BUT, I've never heard of ISP's doing this with modems, so I'm stumped. Send a screenshot to your ISP of the graph, and ask for their suggestions. Maybe they just give modem connections a lower priority, and so the effect is the same?

If you want, PM me with a magnet of the file (control click the file in the library to get the magnet link) and I could let you know what happens if I try downloading it.