-v -legacy cpus=1

Put the above kernel flags into your Boot.plist, to automatically set them.

If you don't know how to do it, check this tutorial -> http://forum.insanelymac.com/index.php?showtopic=16734

<key>Kernel Flags</key>
<string>-v -legacy cpus=1</string>

Then reboot without setting any additional flags, and try to download some files...

This is really just a temporary solution,
I don't think that you only want to use one core instead of two in the long run?

Greetings