Files with the .part extension are a file that are 'in process' of being downloaded. If the download fails, the download Might be resumed from the previous attempt.

This download manager works separately from the browser. The browser tells it what to download, and off it goes.

http://www.freedownloadmanager.org It is a powerful modern download accelerator and organizer for Windows and Mac.

It is possible that (anti-virus) security software is causing the problem. Try to disable security software temporarily to see if that makes a difference.