I got the exact same problem here... did you find a solution to it?
I am also using it in Parallels with XP installed under OSX Mountain Lion 10.8.
Video Memory is set to 1024MB with Enabled 3D acceleration and vertical synchronization.
The system is ...
iMac 27-inch, Late 2009
Processor: 2.8 GHz Intel Core i7
Memory: 16 GB 1067 MHz DDR3
Graphics: ATI Radeon HD 4850 512 MB