I figure I was going to use Shark and figure out, where exactly the time was lost. But I didn't make much progress.
I have to admit that the PMC registers in the PPC 970 are just slightly :) beyond my comprehension. They seem to differ much from the 7450, where I have the documentation. Unfortunately the documentation for the PPC 970 monitoring registers are supposed to be described in detail in Book IV of the architecture guide, of which three volumes have been published...
If anyone has some more info about the PMC registers, please leave a comment or mail me.