System overload? How do I spread load over cores

I'm running Logic 9.1.5 on a 12-core Mac Pro (OS 10.7.2). Why is it that the 12th core of the CPU/HD monitoring window is spiking and causing "system overloads" when my sequence is just not that processor-intensive? For the first time since having this cpu, I'm finding that a single instance of OMNISPHERE with 4-5 instruments loaded is causing that one core to spike and the sequence to overload. When I bypass it, everything's fine. This has never happened before on this machine. Sure- on my Dual 2.5gig G5 it did, but not this thing.

Some people talk about spreading the processing load over the cores more evenly. Is there a way to do this? Some people say there's no answer to this. Spectrasonics said it's not on their end, and that it's the host- in this case Logic. Would love some help! Thanks-