I'm not the best person to answer this, but check out the recommended settings here:
http://return.mistymoorings.com/cirp/index.php#gI noticed you had a few different selections from the recommended, which can cause some mesh issues.
However, it sounds like a performance issue. FSX is terribly optimized, unfortunately, and not really supported officially by MS any more.
I've got a hex-core processor @3.33ghz, 6gb RAM, with an ATI Radeon 5800 for gfx, and I still get bad framerate hits quite often, and "popping" textures. I can run every other modern release (BF3, MW3, Skyrim, etc.) all on ultra detail with no problems.
How many cores is your processor running? Your RAM is a little low as well, if you've got anything else running while FSX is going, you're going to take a hit on performance most likely.