It turns out there may have been several sources for the problems. Part of the issue was not having SR1 on my Poser 5 install (that was causing the "jumping rooms" problem and the dreaded Forearm Issue), but it looks like the instability and system crashes may have been the result of an incorrect bios setting. The friend who built this system for me had the motherboard performance on a turbo setting, which Soyo doesn't recommend. I reset the bios options to their fail-safe defaults and the crashing issue seems to have been resolved. Much thanks to everyone for the help and suggestions! And for the record, I've bought every release of Poser since P1, and I LOVE Poser 5. :)