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*