I'd like to suggest maybe a bad RAM module.
I don't know at all how Mac deals with its hardware, but I once had a bad RAM module that only had certain memory addresses or sectors or whatever that were messed up. Windows failed to detect it and the only way I was able to figure out the problem was by running Memtest 86, using all the deep stress tests.
The symptoms were seen in inexplicable weirdness in programs using lots of RAM. Not Poser, but Zbrush and Softimage. I replaced the faulty RAM and all the problems stopped.

Might be worth looking into at least, if you can find a free Mac utility that does a deep stress test for RAM problems. In the case of my problem, the RAM wasn't maxing out, so it didn't matter if it was enough or not - it was only when it tried to use certain memory addresses I suppose.

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