This happens to me too. It stops at 99% and says "Network problems". Then JD waits 5 minutes, tries to resume the download, finishes it but the MD5-checks fails. After all parts of that package are done, JD tries to unpack it, but fails. I have to manually extract the parts (with WinRAR) to identify the failed part and reset and redownload it again. This problem occurs randomly in one out of roughly 20 downloads, I think.

It could be of course something with my network but as ninguno has the same problem it could be JD as well.