Hey,
This one has been discussed here many times, but because you look like you're new, I'll let you off the hook
Find yourself a copy of Backup2ws. I have had the same problem over and over again with trying to back up over 3gb of data. I think the problem might be to do with browser limitations of a very large file coming in at a very high speed?? just a guess...
Backup2ws is very easy to use, just do a quick search for the how to and once installed all you have to do is open your Admin Panel and then tell it what to back up and to which share and it'll go away and do it.