Whenever you have default items mixed with an addon scenery, the problem is never the addon scenery itself, but it's always caused by another scenery in conflict. That's why you cannot fix it by reinstalling KIAH, since the problem is not KIAH, but another conflicting scenery.
The most common causes for conflicts are the AFCADs ( .BGL files ) supplied with AI Traffic packages like TrafficX, etc. You must remove the affected file if you have an addon airport.
You can use the free FSX Airport Scanner utility to search for AFCAD duplicates.