There are three possible reasons for a flight to end suddenly in my FsX Dx10:

1. Blue Screen of Death...usually overheating from too much overclocking of the cpu or the heat-sink need cleaning. Occasionally it can be a ram card or socket issue..

2. Crash to Desktop... vas usage up to the 32 bit limit. I have FSUICP installed and the audible warning tells me to save the flight before the CTD.

3. Lock-up of the visual with the sound still looping. This is what I suffer most with 1 and 2 above being (fortunately) a rarity. Sometimes the lock-up lasts for just a few seconds and occasionally I need to reboot to clear it.

I have yet to find an explanation for type 3 and can only think it may be a product of insufficient video ram. I will be updating my card soon and I'm wondering if a 4 or 8 gig Nvidia card will help?