Well there could be a number of reasons for the crash but the g3d.dll is the visual end of the bgl converter, when its overloaded or it receives a response its not happy with it causes a crash.

Im starting to wonder if it caused by a duplicate GUID issue between the stock scenery and addon scenery, when there is no stress on the g3d.dll it can cope with the issue but when you up the settings and run the sim on its edge it can't deal with the issue fast enough and causes the crash. Just a theory mind you.

You have been very very lucky not to have a g3d crash Markoz :)