In that case you need to figure out what is requesting memory from Windows such that it's paging SQL out. It'll only do that if it is under memory pressure (the OS) and SQL is not responding fast enough in reducing memory.
Probably will need a lot of time looking at perfmon, the process object, the memory counters for all the processes.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability