Disable UPnP and select your own port, when I started out with Limewire which I purchased from the official site I had the same problem, but disabling UPnP in the advance firewall settings of Limewire seem to do the trick.

Another tip, too many searchers and downloads can also increase cpu usage, I now limit downloads to 5 at a time, your machine is much faster than mine so I figure you could do 8 at a time, maybe.