Pain and Virtue

I am having a similar problem, i have a 21.5 Imac, 10.6.4, ATI Radeon HD4670. X-Plane loads fine, but as soon as you start to take off, it goes into a black screen, and i need to reboot the mac. Anyone have a solution. I spoke with Apple technical support, and they hadnt heard of the problem, asked me to run tech tool to check the video ram, which i did and it passed..... Driving me nuts. Emailed Laminar and havent received a reply. Any help would be gratefully received. Andrew.