Are you talking about doing an Update within QB?
I think, but am not 100% positive, that update downloads are interruptible when using the Update feature within QB. If interrupted it will pickup again where it left off. This may vary by the version of QB you are using. If you are just trying to download the manual update file, I don't think that is interruptible, but again I am not positive.
Overnight is probably your best bet. You said that your connection is occasionally interrupted. Is this because you have Call Waiting? If so, disable it before calling. If not, just try it and see what happens.
Another option if you want the manual update file is to get someone you know who has a fast connection to download it for you and copy to a CD.
Hope this is of some help.