Does it only happen when Num is rewired/running?
Did you try updating OSX/Live/MOTU/Num3?
Also, go to Terminal, type 'top' and monitor what process is actually spiking the CPU. Might not be related to audio.
Addendum: I seem to remember the new i7's having temperature & threading issues related to speed-step & firewire interfaces. Try googling around.
http://www.gearslutz.com/board/music...deo-proof.html
FWIW: I'm on a very similar setup (1st-gen Intel MacPro Desktop, OSX 10.6, MOTU 828mk2, Live 8, Num3 w/LP) and haven't seen what you described.