I recently came across a small problem with downloading large files (using flashget or jdownloader). When extracting rar files, i was getting alot of 'CRC Errors - file is corrupt'.
After changing nic card, ethernet cables & so on, the simple problem was an update in AVG which was causing the problem.
By turning AVG's Link Scanner off will solve the problem.
Hope this helps anyone, as it took me days figuring it out.