Using SQL Server 2005 Standard Edition 9.0.1399

My msmdsrv.exe is not releasing memory and not reusing it.
From Process explorer this is what I see .
when usage increases the Working Set of msmdsrv.exe increases along with
Private Bytes. When usage reduces Working set goes down but Private Bytes
remains the maximun used memory.
Later when usage again increases Working Set keep growing. this time Private
Bytes also grows .
For example msmdsrv.exe)

Working set went to 6 GB , Private Bytes also to 6GB. Task Manager shows 6
GB memory usage ( msmdsrv.exe = 6 gb)

Working set went to 2 GB , Private Bytes is still 6 GB. Total Task Manager
memory usage shows 6 GB memory usage ( msmdsrv.exe = 2 gb)

Working set went to 8 GB , Private Bytes is 6+8= 14 GB. Total Task Manager
memory usage shows 14 GB memory usage ( msmdsrv.exe = 6 gb)

In this case server starts paging for additional RAM requirement .
Not sure why msmdsrv.exe Private Byte show all these RAM.
Anybody faced this issue or got any idea/solution.
Any help is much appreciated.

Thanks
Binu