If you don't see just SOME buildings, it looks like you have some video memory corruption or exhaustion, because of too high settings.
Try all the following:
- Update your video driver
- Reset your video driver settings to default
- Reset the FSX settings to default, by removing the following file:
%APPDATA%\Microsof\FSX\FSX.CFG
This will force FSX to rebuild a new config with all default settings. After doing this, be sure you apply the HIGHMEMFIX tweak in the Addon Manager, save the settings and restart FSX.
If still happens, maybe you are simply used all your available Video RAM, but it's difficult to say without knowing your system and how many add-ons you have installed. In any case, this will be fixed by just lowering your settings.