May just be a case of overly high settings but as the OP have said, there are zillions of possible reasons.
Try moving the sliders down a bit and see if it still happens.
Could be an aircraft or a piece of scenery you have installed (try not using/disabling).
One of my recent problems turned out to be related to my default flight, so deleting that is next on my troubleshooting list after a rebuild of the fsx.cfg.
Could be a setting you have made in your videocard software.
Are you keeping an eye on your temperatures?
Any tweaking you have done to fsx or your system in general could be to blame. Default your settings and see how that goes.
These are only a few of the possibilities which can seem endless I'm afraid, but hope one of these turns out to work for you.
Q9550 @ 3.78 GHz with Gainward GTX570 1.25 Gig DRAM
4 Gig DDR2 RAM - Windows 7 64 Bit
Resolution 1680 X 1050 X32