Interesting post here on the PSS SQL Server Engineers blog on SQL Server working set trim problems:
There’s also a related KB article here:
And a representative blog entry on the subject here:
I’m working with a customer where PSS think this is happening with Analysis Services. Certainly Perfmon shows some massive drops in memory usage for no obvious reason (ie the system is nowhere near the LowMemoryLimit). Something to look out for… I’ll post more details if/when I get them.