Some torrent files may confuse 
KTorrent and it may go into an infinite loop consuming all CPU cycles but essentially doing nothing.  I had a similar experience just a few hours ago.  How did I solve it?  I used the command 
ktorrent --debug to see what it does.  Reading the debug output helped me find the directory* where KTorrent stores the current torrent files' information.  I just removed the directory of the torrent that was causing trouble.  Voila, KTorrent is back again to normal!
[*] 
$HOME/.kde/share/apps/ktorrent/tor*