Safari keeps losing connection
If you are downloading large files like this on a connection that isn't reliable you're best bet is probably to use a command line tool like curl.

Open a Terminal window, change directory to wherever you want to download the file (for example cd ~/Desktop to save to your desktop). And then type the command curl -OC - http://isoredirect.centos.org/centos/7/isos/x86_64/CentOS-7-x86_64-DVD-1503-01.iso to download the CentOS ISO image (I just picked this URL for an example, use you own as you wish).

If the download gets part of the way and stalls or dies, you can just run the same command again and it should restart from the place it left off.

The O in the command tells curl to use the remote filename and the C - tells it to continue where it stopped previously.

Please note: servers have to be set up to allow downloads to start from random positions, so if you try this and it always starts from the very beginning it could be that the server doesn't support it.