[tonebloke]

Try using a different converter. I've had the odd problem when using TLH and dbPowerAmp.

As far as re-doing the files:- I use Utorrent. This may work for you.
Delete the torrent from Bittorrent; NOT the "Torrent and Data". You want to keep the Data. Then delete the files that you think are corrupt from the designated folder. Download the link again and point it to the folder you already have. It will recognise the files that are left in the folder and start downloading the missing files.
Lots of luck!!