Increase your server RAM.
Check your network. At times windows 7 makes problem.
My best experience with windows XP. 13 simultaneous users.
Better to use a dedicated server for BD alone if possible.
If u r using BD ultimate not to use BD SDK as your server instead create an application and use that application as a server and control your records through user groups.
Certainly server version is good enough.
However think of when you create application you are coining only the components what you require and you are going to use that only for its specific DB.Hence possibility to get unwanted error from the software is less. But when you use your compiled application you lose some facility like user status cannot be known from server, optimization etc.
My personal experience is created application is best.