I'm having the same issue with the 32-bit version of the software. When I have some elements selected and try to move the view port it will jump around from perspective to orthographic and back again. Sometimes the view port will even go completely black.
Running...Windows XP 32-bit, 512MB GeFroce 8800GT, AMD X2 4200+, 2GB Crucial DDR2, 160GB Seagate SATA II