I had a problem like this in the dark old days (early last year ).
It all boiled down to two things.
ONE: Make sure all imported models have NO open meshes (MED mesh tester)
TWO: Try to avoid walls that are too thin. If the wall is less then 8 quants,
either make the wall thicker, or scale the whole world up a notch till it is.

PS - you'll need POLYGON = ON for the houses, and preferably for the player too.