I have the same problem as well. Always on a long flight. Therefore I created a KJFK to KLAX flight that I have repeatedly flown approximately 12 times over the last few weeks for testing. I have never successfully completed a flight with the MD11 or the 747 without a black screen so far except when I used DX10. Other points:
Default 747 was successful on KJFK/KLAX without a black screen using DX9.
One flight I had menu corruption. AI aircraft were added sequentially to the menu to the right of the PMDG menu entry (screenshot avail. :-))
FSX Config settings such as TEXTURE_BANDWIDTH, FIBER_FRAME_TIME, TEXTURE_RESOLUTION etc have had no effect.
Autopilot begins to behave erratically before the problem (Can't maintain track even on 1x speed - winds aloft disabled).
System Specs:Intel Core2 6700 @ 3.0 Ghz4 Gig MemoryVista 64Nvidia 8800 GTX SLIFSX Addons: UTX, FEX, TrafficX