Yep, I"m having the same problem. What I do is bring "task manager" up by simoutaneously pressing "alt+ctrl+del" than click on start task manager.
you see, flight simulator doesn't shut down, it just minimizes. What you have to do is maximize it after you find out what caused the minimizing. Once you
have brought up task manager it show the application programs you are running. You may have your printer on and running or any other major program
runnung also. You need to remove all the other programs so that your computer is working FSX only. To do that you right click the item to remove, what
comes up is a function screen, left once End Task. When done taking them out, right click FSX than left click MAXIMIZE. FSX screen should come back
up in the pause function. Thats how I deal with it, and it works for me.