I remember there being a program that downloads and stores what has been downloaded should the connection fail. When the connection returns, it picks-up where it left. I'm not sure if it will work as the download goes through I-Tunes.
There are many download managers available (both free and paid-for). Some are stand-alone programmes and others integrate with your web browser. As browsers up-date, some won't work, so it's important to check that the one you want is compatible with your current browser.
For example: I have several different ones. In my Firefox version 7.01 I use DownThemAll.
Have a look at some of these:
DownThemAll!
Best Download Manager - FlashGet
Free Download Manager - absolutely free download accelerator and manager
I don't use iTunes so am not sure if they'll work for you. (You didn't say if your computer is an Apple or Windows-based, or which version.)
If they don't, I suggest you do a Google search for "download managers", and maybe "iTune downloaders".
Hope this helps.