What were you flying ?
I've had the same problem with newer addons that were developed specifically for fsx - the PMDG J41, and the F1 Mustang. Apparently it's because they are so complex that while windows might physically have enough ram to run them it doesn't have enough memory addresses to remember everything. The solution seems to be either to reduce the compelexity and thus how many addresses are needed (aka turn something down) or move to a 64 bit version of windows which can handle rather a lot more memory addresses than a 32 bit version (aka turn something up).
One other thing that has been suggested is this, I don't know how good the suggestion is but here it is : Run in windowed mode and have the task manager open as well somewhere you can watch how much ram fsx is sucking while you are using it. Keep an eye on that memory figure, every time you change to a different view and it has to load new textures apparently the memory usage goes up. Never down. always up. Eventually the inevitable will happen, keep the ram that was in use at the time in the back of your head. Now repeat the exercise, still in windowed mode, but when the ram usage starts to get up there minimise the fsx window then restore it and keep flying. You should find that minimising it dumped all the stored textures and reset the ram usage, and so long as you remember to do that periodically before it gets to the point at which fsx laughs and shuts down you should be right.
Personally, I'm picking up my copy of Win7 HP 64 tomorrow afternoon ........ Itr's still close enough to the purchase of the beast that I can count it as part of the cost of upgrading

Gary
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