http://www.gnutellaforums.com/downlo...wards-why.html gives a probable explanation.

If you have more than one disk partition or even better another internal disk drive, move your downloads across via your settings. Make sure you defragment that drive before you do that however. How to move them across? Go to LW's menu bar, Tools -> Options -> Downloads & change location. With LW 4 from memory you could change location of a download half-way through, I'm not sure if this is possible with LW 5 but worth a try. Your issue could be due to a number of reasons. The link I gave suggested problems with confirmation between the original hash file & the actual file downloaded. But there might be other reasons such as a highly fragmented drive on your computer you are downloading to. And p2p sharing programs such as LW do fragment drives very very quickly. So if possible, move the downloads to a drive/partition where fragments are small, & no operating system also requiring use of same disk drive.

An over strong firewall/security software or an old unreliable router will also have same effect. (ie: router not parsing data correctly or reliably. Or a security program that is affecting the data transfer or damaging the data.)