Unity 3.5.6 occlusion culling visibility problem

I have a Unity Pro 3.5.6 project that is near completion. I converted from the 3.5.6 free version to Pro last month and had a significant performance reduction. I have a 5.x license but am not able to upgrade to that at this time due to time constraints.

I added occlusion culling to my project to restore/improve performance but have run into a problem. The details are as follows: the system appears to not show visible objects (e.g. walls, floors) in some cases; these should be visible due to camera position; large black areas appear instead. I am using Automatic Portal Generation but also tried PVS Only and set flags (occluder/occludee) properly as per documentation. When I set View Cell Size below 8 I get the problem. When View Cell Size is set higher (e.g. 8) and rebake the problems seem to disappear but performance is not good; at all lower View Cell Size values the problem reappears.

Please let me know if upgrading to 3.5.7 would solve this type of problem or otherwise any technical suggestions with respect to this issue would be greatly appreciated. Thanks!

OK, I am going to answer my own question in case it might help other developers. I discovered a solution. You have to attach a custom Occlusion Area to the camera in each scene. The area center is (0,0,0) and width=length=2*camera.far_clip; height is related to the size of your occluders. Seems like there are known issues with UMBRA occlusion culling; in my case using the default occlusion area was the problem.