Recently i have come across alot of file servers which have a large amount of in use and because of this are slow.
When you investigate the task manager you see that there is little or no services taking up RAM but yet you are hitting the memory limit.
The next step for me was to use a System Internals Tools called RAMMap which identified that the Metafile was nearly 3gb (in my case) which is massive!
There is apparanetly a known issue in Windows which causes this metafile to grow on a windows file server.
To help manage the problem more effectively so it doesnt kill the server altogether check this link out http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=9258
This tool will help you manage the Dynamic Cache on the server allowing you to set limits to it doesnt have free rain over the server.
Hope it Helps!