Usually these type of problems are not easy to solve, because Max doesn't any error. I would try several rendering tests, using only the main workstation and including different groups of objects (only trees, trees and terrain and so). Once you have the minimal scene where the problem is reproducible, sould be much more easy to solve it.
If you send us the scene, we can take a look to it. If the file is too large, please tell me and i will provide you a ftp account to upload it.