Well, the 8800GS in the iMac is actually a 512MB 8800M GTS. Apple just rebadget it, since the performance is closer to a 8800GS, as the mobile card is clocked lower compared to the desktop version. However, I do not think it is a heat problem, as the problem occurs right within seconds into the flight (if I trigger it), and no matter if the fans are running at 2000 or 6000rpm. Also, Crysis runs flawlessly for hours while putting the 8800 to work a lot more than FSX.

Bob, on a side note, some addon sceneries do this because they have no mip mapping when they are shipped. I noticed that a lot of airports I buy from Aerosoft don't look right until I manually add mip maps. I am not saying that this will cure your problems, but even on my system that is set up properly I will get some jaggy lines in some sceneries that aren't or have no mip maps. If you have texture files that contain .bmp files you can add mips to those using imagetool, or if they have .dds files you can check them and add mips using dds converter. You can also do the same to aircraft that are shipped without mips using the same tools.

I read in NickN's FSX tuning guide that this is a problem with DX10. Try running with just DX9 and using Nhancer. Here is the link to Nick's tweak guide, you should go through all of it and see how it works out.http://www.simforums.com/forums/forum_posts.asp?TID=29041