I am still puzzled about Inverness. I have just conducted a quick test flight at the airport in the (admittedly VAS hungry) PMDG 737NGX, and I experienced quite a lot of those "black rectangles" on the terrain as I was on the downwind leg, and turning onto final approach. I can't understand why this should happen at Inverness, because it is not a particularly dense scenery area. I tried the test again with the freeware FSX Power Project electricity pylons enabled, and I experienced large expanses of white rectangles where terrain textures should have been, and all the autogen disappeared! In fact, my entire 737NGX VC disappeared at the same time! I am amazed that FSX was still running!!
I am starting to wonder whether this problem is related to the "invisible approach light support structures", since I have never actually seen them yet!
This is with the 512x512 textures installed.