I don't think you can fix it. As far as I know, it's a fault with FSX. The ground/scenery textures are "floating" above the ground in some areas, especially if the ground has a lot of small hills or mountains (the ground is not flat, but the textures are).

I shot down a 737-800 near YMML not long after I got Weapon for FSX, I did not hit it enough times to destroy it (I ran out of missiles ::)) but it continued on its merry way to wherever it was going ON THE GROUND it kept disappearing below the surface as I followed it far enough away to no longer be able to track it. I think it was trying to go to New Zealand. It was really funny to watch it flying along the ground!

Mark