This behaviour of LW could have several reasons.
For instance, a corruption of the preference folder, which could be dealt with by simply deleting it as described in Point #2 & 3 in Fixes for Limewire:
http://www.gnutellaforums.com/showth...threadid=36420
Also a possible a Java issue, which could be solved by upgrading the present Java installation to the newest version. To do so, you should uninstall all previous versions, reboot, download the newest version using the JRE offline installer from here:
http://java.sun.com/j2se/1.5.0/download.jsp
and install it and reboot again.
In case this problem occures with files supposely offered by many hosts (20 up to hundreds) then this problem is not to be solved since those files are usually faked ones hosted by people disliking P2P file sharing.
Using the Bitzi function can help minimize to risk of trying to download such faked files:
http://www.gnutellaforums.com/showthread.php?t=37676
Last but not least, it is also possible that your ISP has started to shape (throttle) the bandwidth you're using. A test download of a magnet link from here could give you some indications whether this is the case:
http://www.magnetmix.com/video.shtml 