You have a very strange problem indeed. I have no idea what the problem might be, but I can only think that something you may have installed 18 months ago, may be affecting FSX. Whatever it was you installed, you install when you upgrade or buy a new computer. But that is only what I think might be your problem. :-/

I strongly suggest that you go to MS FSX Forum, over at AVSIM (you will need to register/login) and ask for help there, as you are more likely to find others who may have had a similar problem to yours, or who may be able to provide a solution.