1. PostgreSQL 8.3, located on the same host - Windows Server 2008 R2 Datacenter
2. Yes, subnet is the same. Both servers(OF and DC) are virtual, esxi 5.1
3. At about 200 simultaneously connected users and more then 2000 users in AD.
When some people have the problem while the others don't, it can be error in configuration, but it also can be a bug.
In my case it works partially. I have 2 disappearing groups, while the other groups works fine, so i think it's a bug.