Why do my downloads start and stop (and often not finnish) when I am downloading larger files like movies. I tend to choose downolads with multiple users and high speed connections. Songs seem to work fine.

Torrent files right?
Either way, it sounds like those online at the time do not have the entire video, else those with the entire video have their upload slots busy with others. So you can only download as fast as someone else who is downloading the same video & perhaps even slower than them.