All users that ever had this problem confirmed it was always fixed by either resetting the video drivers to default, or resetting the FSX.CFG, adding the HIGHMEMFIX tweak.
But of course, with FSX all bets are off. As a 32 bit program, with not even all the whole 4GB of RAM available to apps, and the more VRAM you have, the less normal memory the apps have ( because in DirectX 9, part of the video memory is mapped inside the normal address space, stealing away memory for the sim) so, it's very possible that, with your addon and settings, you reached the limit in which FSX can't no longer display all graphic without some kind of memory corruption.
Really, having a modern video card like yours, with FSX, it's probably the worse possible choice: FSX is not fully using it ( it's basically wasted, since FSX is not really GPU-dependant ), but its considerable amount of memory is stealing away resources for the simulator so, the more objects and textures the sim use, the more memory is stolen from from FSX, so it's the worse possible case, you are not using your card, because FSX doesn't use it much, but because of it, the sim has less memory to work with, so it's more at risk of crashes and, even if it doesn't crash, when it's close to memory exhaustion, it will start showing strange things.
In addition to that, if you are not using the Steam version or the Acceleration Pack, and are still using the old Service Pack 2, there are bugs in it ( which nobody will fix, of course ), which cause display problems that don't happen with the Steam version.
So, if you are not using the Steam version, that might probably the cause.
If you are, then you should really consider switching to a P3D4, which will use your hardware much better, with none of the issues FSX has as a 32 bit app.
