You're trying to download the exact same files right? I've had that problem with particular files. Each time I'd end up with the same corrupt file no matter which host I tried to find. However, I know eventually I did find a decent copy of one at least. Why this happens, I don't know. Perhaps due to the uploader's firewall/security, or the file they have is corrupt or an ISP in between is somehow affecting things.

Is the fault at the user's end? It could possibly be ... an over aggressive AV program or router that is not parsing data properly (this could be happening at the uploader's end too.)

I do remember one suggestion for corrupt files was to delete your preferences folder. I'm not sure this will fix things every time however.

And yes, download % dropping back is related to a corrupt file message. However if you clicked yes to continue & clicked the option to always use that answer, then you would not see that pop up window again, thus seeing files go backward in % might take you by surprise. You can revert popups to default in your settings.