Hard to say what it is but sounds like an internal problem with windows,
try a few things.
1. Reboot your computer. If you go long periods with your computer constantly on, it seems to develop problems.
2. Run Scandisk and defrag
3. Use some type of registry cleaner. I use System mechanic 6 Pro that includes many tools that detect and repair errors. You may have trash in the system registry that is causing problems.
4. Check to see how many items are running at start up. Using the Run selection, type msconfig. It will show items that are set to start automatically during start up. Too many unnecessary items can eat up your RAM. You may be at the limit of your available RAM, more RAM can only help.
5. Try turning down your settings in FSX.