Filled in half the details. Any chance of completing this: System details - help us to help you (click on blue link). There might be something you are missing. Modem/routers are sometimes a problem if UPnP is not functioning properly or their implementation is unreliable. Are you running wired or wireless?

Roughly how many incomplete files do you have? (Explore the incomplete folder from the Library window, use the folder description to find out how many files)