-go in zoneview and look wich brush could make a bad bsp cut, then move the bad brush 4 units inot another direction and look again, or make it semi or solid
-export total map, and import again
-rotate and move total map 90 degree and rebuild
-buy a new pc
and if the invisble wall oly occurs aftera full rebuild, and not aftr an only geometry rebuild, try doing seperate rebuilds then, so first and only geometry, then light, then bots, then save