Maybe you're seeing some corrupted files. Does this happen with every large file that you try to download?

When the download % drops back, it can mean there's some corruption within the file...the thread below explains more.
Downloads "Jump" backwards

Check files with Bitzi before download. People use Bitzi to rate files, so checking before download can give you some idea of the content & quality of the file.
How to use Bitzi

Also have a look at LOTR's post below.
what does it mean if the file is corrupted?

Maybe your firewall or one of your security programs is causing problems?
Are you seeing the firewall icon (brick wall in front of the blue globe)?