The objects in question may need there LOD fixed
or at least LOD's created,there should be three levels of detail for each object
although some have only one level of detail
The T.O.W. objects(like the sidewalks you pictured)
do not have proper LOD's made for them,and is why they disappear at certain distances
do to not having a level of detail created for that distance
Fix the object correctly,the problem would go away